U.S. patent application number 12/580505 was filed with the patent office on 2010-04-22 for projection electron beam apparatus and defect inspection system using the apparatus.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. Invention is credited to Ichirota NAGAHAMA, Yuichiro YAMAZAKI.
Application Number | 20100096550 12/580505 |
Document ID | / |
Family ID | 36941298 |
Filed Date | 2010-04-22 |
United States Patent
Application |
20100096550 |
Kind Code |
A1 |
YAMAZAKI; Yuichiro ; et
al. |
April 22, 2010 |
PROJECTION ELECTRON BEAM APPARATUS AND DEFECT INSPECTION SYSTEM
USING THE APPARATUS
Abstract
A sample is evaluated at a high throughput by reducing axial
chromatic aberration and increasing the transmittance of secondary
electrons. Electron beams emitted from an electron gun 1 are
irradiated onto a sample 7 through a primary electro-optical
system, and electrons consequently emitted from the sample are
detected by a detector 12 through a secondary electro-optical
system. A Wien filter 8 comprising a multi-pole lens for correcting
axial chromatic aberration is disposed between a magnification lens
10 in the secondary electro-optical system and a beam separator 5
for separating a primary electron beam and a secondary electron
beam, for correcting axial chromatic aberration caused by an
objective lens 14 which comprises an electromagnetic lens having a
magnetic gap defined on a sample side.
Inventors: |
YAMAZAKI; Yuichiro; (Tokyo,
JP) ; NAGAHAMA; Ichirota; (Tokyo, JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW, SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
Tokyo
JP
|
Family ID: |
36941298 |
Appl. No.: |
12/580505 |
Filed: |
October 16, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11817763 |
Dec 8, 2008 |
|
|
|
12580505 |
|
|
|
|
Current U.S.
Class: |
250/310 |
Current CPC
Class: |
H01J 37/28 20130101;
H01J 2237/057 20130101; G01N 23/2251 20130101; H01J 37/05
20130101 |
Class at
Publication: |
250/310 |
International
Class: |
G01N 23/225 20060101
G01N023/225 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2005 |
JP |
2005-059504 |
Mar 28, 2005 |
JP |
2005-092297 |
Mar 28, 2005 |
JP |
2005-092314 |
Claims
1. An electron beam apparatus having a projection electro-optical
system for inspecting a surface of a sample, comprising: an
electron gun for emitting an electron beam; a primary
electro-optical system for guiding the emitted electron beam onto a
sample for irradiation; a detector for detecting electrons; and a
secondary electro-optical system for guiding an electron beam
bearing information on the surface of the sample, emitted from the
sample irradiated with the electron beam, to the detector; wherein
the secondary electro-optical system includes a Wien filter having
two focus points.
2. A defect inspection system for inspecting a defect on a surface
of a sample, comprising: an electron beam apparatus having a
projection electro-optical system, as claimed in claim 1; an image
capturer for generating an image of the surface of the sample based
on information on the surface of the sample included in electrons
detected by the detector of the electron beam apparatus; and a
defect evaluator for testing the presence or absence of a defect on
the surface of the sample by comparing the captured image with a
reference image.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. application Ser. No.
11/817,763, filed on Dec. 8, 2008 which is based on Japanese Patent
Application No. 2004-306641 filed on Oct. 21, 2004 and Japanese
Patent Application No. 2005-032157 filed on Feb. 8, 2005, and which
is based on and claims priority of Japanese Patent Application No.
2005-059504 filed on Mar. 3, 2005, Japanese Patent Application No.
2005-092297 filed on Mar. 28, 2005, and Japanese Patent Application
No. 2005-092314 filed on Mar. 28, 2005, the entire contents of
which are incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a projection electron beam
apparatus and a defect inspection system using the apparatus, and
more particularly, to an electron beam apparatus which has a
projection electro-optical system to have the abilities to conduct
a defect inspection and the like for semiconductor wafer and the
like at a high throughput, and a defect inspection apparatus using
the apparatus. For example, the present invention relates to an
electron beam apparatus which evaluates a sample having a pattern
with a minimum line width of 0.25 .mu.m or less and preferably 0.20
.mu.m or less at high accuracy and high throughput, and a device
manufacturing method using the apparatus.
BACKGROUND ART
[0003] Generally, in an electron beam apparatus which is configured
to irradiate a sample applied with a retarding voltage with a
primary electron beam and detect secondary electrons emitted from
an irradiated spot to produce an image of the sample, a larger
electric field must be applied between the surface of the sample
and an objective lens in order to reduce axial chromatic
aberration. However, as the electric field strength is increased on
the surface of the sample with the intention to reduce the axial
chromatic aberration, a discharge will occur between the sample and
objective lens. For this reason, the electric field strength must
be set to be relatively small, resulting in a problem of relatively
large axial chromatic aberration.
[0004] Also, an electron beam apparatus is known (see, for example,
JP-A-2004-214044) for shaping an electron beam emitted from an
electron gun into a rectangular shape, irradiating a sample with
the shaped electron beam, producing an enlarged image from
secondary electrons emitted from the sample using a projection
optical lens, and detecting the sample image using a detector such
as a TD1 detector. This apparatus employs an electrostatic lens as
an objective lens for irradiating a sample with an electron beam
(see, for example, JP-A-11-132975, Republished WO2002/045153).
Also, a secondary electron projection optical system is known to
employ an electrostatic lens having five electrodes, and further in
a lithography apparatus, an electro-optical system which satisfies
a MOL (Moving Object Lens) condition is also known.
[0005] When an electrostatic lens is employed for an objective
lens, a higher voltage applied to a central electrode of the
electrostatic lens results in a higher susceptibility to a
discharge. Accordingly, when a low voltage is employed, axial
chromatic aberration increases. When the electric field strength is
increased in order to reduce the axial chromatic aberration, a
problem arises in that a discharge occurs between a sample and the
lens. Anyway, the transmittance of secondary electrons is difficult
to be increased, as a consequence.
[0006] The employment of a magnetic lens for an objective lens is
also known (for example, see JP-A-2003-168385 and
JP-A-2003-173756). In this event, problems lie in that an axial
magnetic field is not zero on the surface of a sample, and
secondary electron beams emitted from the sample in the normal
direction do not intersect with the optical axis or pass through an
NA aperture. Also, a MOL scheme has problems that a beam must be
scanned, and it is hardly compatible with an inspecting device.
Further, in a rectangular visual field, problems are a long
distance from the optical axis for the area, and large astigmatism.
Another problem lies in that a second electron beam is blurred by
space charges of a primary electron beam.
[0007] From the foregoing, a need exists for an electron beam
apparatus which is free from a discharge occurring between a sample
and an electrostatic lens, can reduce axial chromatic aberration
and various aberration, can force secondary electrons to intersect
with an optical axis and to pass through an NA aperture, can vary
the magnification of an image by secondary electrons, can improve
the transmittance of the secondary electrons, and can reduce the
occurrence of blurs of the secondary electrons due to a space
charge effect of primary electrons, and for a semiconductor device
manufacturing method using the apparatus.
[0008] Conventionally, when an LaB.sub.6 cathode is operated in a
space charge limiting region, there is an advantage of small shot
noise, but on the contrary, there is a problem of large chromatic
aberration due to a large energy width. A technology for correcting
an objective lens for axial chromatic aberration using a plurality
of quadrupole lenses in order to accomplish a high resolution of
several nm to one nm has been practically used in SEM and
transmission electron microscope.
[0009] On the other hand, with the trend of higher integration of
semiconductor devices and increasing miniaturization of patterns,
inspection apparatuses have been required to provide higher
resolution and higher throughput. In order to examine a wafer
substrate of 100-nm design rule for defects, it is necessary to
view the presence/absence of pattern defects and particles in wires
having a line width of 100 nm or less, defective vias, and electric
defects thereof. Accordingly, a resolution of 100 nm is required,
and a higher throughput is also required because the amount of
inspects is increased due to an increase in manufacturing steps
resulting from higher integration of devices.
[0010] As devices are formed of a larger number of layers, an
inspection apparatus is also required to provide a function of
detecting defective contacts (electric defects) of vias which
connect wires between layers. It is anticipated that an electron
beam based defect inspection apparatus will go mainstream in place
of an optical defect inspection apparatus in regard to the
resolution and defective contact inspection. However, the electron
beam based defect inspection apparatus is disadvantageously
inferior to the optical one in regard to the throughput. As such, a
need exists for the development of an electron beam based
inspection apparatus which is capable of a high resolution, a high
throughput, and detecting electric defects.
[0011] It is said that the resolution of the optical inspection
apparatus is limited to one half of the wavelength of used light,
and the resolution is approximately 0.2 .mu.m in a practiced
example. On the other hand, in a scheme based on electron beams, a
scanning electron beam scheme (SEM scheme) has been brought into
practical use, where the resolution is 0.1 .mu.m and an inspecting
time is eight hours per wafer (200 mm wafer). In addition, the
electron beam scheme is largely characterized by its abilities to
inspect for electric defects (disconnected wires, defective
conduction, defectively connected vias, and the like), but merely
provides a very low inspecting speed. Therefore, the development of
a defect inspection apparatus which provides a high inspecting
speed is expected.
[0012] Further, a known electron beam apparatus irradiates a sample
with an electron beam having a rectangular cross-section, enlarges
secondary electrons emitted from the sample, focuses the enlarged
secondary electrons onto a detection plane, and inspects the
surface of the sample (see, for example, JP-2002-216694). However,
since this type of electron beam apparatus has large axial
chromatic aberration, the throughput must be largely reduced in
order to provide an S/N ratio required to evaluate at a high
resolution.
[0013] Another known electron beam apparatus scans the surface of a
sample with a plurality of beams, and detects secondary electrons
from the sample using a plurality of detectors to increase the
throughput (see, for example, U.S. Pat. No. 5,892,224). However, in
scanning a plurality of beams, no clear solution has been provided
in regard to how a plurality of beams should be arranged in order
to most effectively perform evaluations. Moreover, the electron
beam apparatus has a problem in that if a magnetic lens is employed
for an objective lens, secondary electrons emitted from a sample in
the normal direction to the surface of the sample do not intersect
with the optical axis.
[0014] Also, while it is known to produce an image at an ultra-high
resolution of 1 nm or less by correcting axial chromatic
aberration, an increase in beam strength has not been practiced,
instead of improving the resolution by the correction of
aberration.
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0015] The present invention has been made to solve a variety of
problems described above, and one of its objects is to reduce axial
chromatic aberration and increase the transmittance of secondary
electrons, thereby permitting an electron beam apparatus to
evaluate a sample at a high throughput.
[0016] Also, another challenge of the present invention is to
provide an electron beam apparatus which shapes an electron beam
emitted from an electron gun into a rectangular shape using a
condenser lens and an aperture, focusing the electron beam on the
surface of a sample using a condenser lens and an objective lens,
producing an enlarged image from secondary electrons emitted from
the sample using a projection optical lens system, and detects the
enlarged image using a TDI (Time Delayed Integration) detector or a
CCD detector to capture a sample image, where the electron beam
apparatus is capable of reducing blurs of secondary electrons due
to a space charge effect of primary electrons, reducing axial
chromatic aberration without causing a discharge between the sample
and an electrostatic lens, simultaneously reducing various
aberrations, allowing secondary electrons or reflected electrons
emitted from the sample in the normal direction to intersect with
an optical axis and pass through an NA aperture, varying the
magnification of an image represented by the secondary electrons or
reflected electrons, and improving the transmittance of the
secondary electrons, and to provide a method of manufacturing a
semiconductor device using the electron beam apparatus.
[0017] While the axial chromatic aberration correction technology
can be applied to SEM and transmission electronic microscope, it
cannot be simply applied to a sample evaluation apparatus and a
lithography apparatus for inspecting defects and the like, which
have a relatively wide visual field. Specifically, a plurality of
quadrupole lenses, if employed, can correct the axial chromatic
aberration, but increase off-axis aberration when the visual field
is wide. Accordingly, even if a plurality of quadrupole lenses are
simply used in a defect inspection apparatus and a lithography
apparatus which have relatively wide visual fields, off-axis
aberration is produced to blur a resulting image. Also, the
lithography apparatus has another problem of a largely limited
throughput due to the space charge effect.
[0018] Accordingly, it is another object of the present invention
to provide an electron beam apparatus which is capable of producing
a beam current larger by a factor of 10 or more at a resolution of
ten to several tens of nm, as compared with the prior art example,
by the action of axial chromatic aberration correction in a defect
inspection apparatus and a lithography apparatus which have
relatively large visual fields.
[0019] Generally, since the inspection apparatus is expensive and
provides a throughput lower than other processing apparatuses, the
inspection apparatus is currently used after important processes,
for example, after etching, deposition, or CMP (chemical mechanical
polishing) planarization process, and the like. An electron beam
based inspection apparatus narrows down an electron beam (the
diameter of which is equivalent to the resolution), and scans the
resulting electron beam to irradiate a sample in a linear fashion.
On the other hand, a region under observation is irradiated with
the electron beam in a planar fashion by moving a stage to a
direction perpendicular to an electron beam scanning direction.
Generally, the electron beam has a scanning width of several
hundred .mu.m. Secondary electrons generated from the sample
irradiated with the narrowed electron beam (called the "primary
electron beam") are detected by a detector (scintillator and
photomultiplier or a semiconductor based detector (PIN diode type)
or the like). The coordinates of the irradiated spot and the amount
of the secondary electrons (signal strength) are combined to
produce an image which is then stored in a storage device or output
on a CRT (Braun tube). The foregoing is the principle of SEM
(scanning electron microscope), and an image captured by this
scheme is used to detect defects on a semiconductor (generally, Si)
wafer which is in the middle of processing.
[0020] The inspection speed (equivalent to the throughput) is
determined by the amount and beam diameter of primary electron
beams (current value), and a response speed of a detector. The
highest response speed is currently 100 MHz for a detector with a
beam diameter of 0.1 .mu.m (considered to be the same as the
resolution) and a current value of 100 nA, in which case it is said
that the inspection speed is approximately eight hours per wafer of
200 cm diameter. This significantly low inspection speed, as
compared with the optical scheme, is a grave problem. Particularly,
for a device pattern of a design rule of 100 nm or less created on
a wafer, i.e., line width of 100 nm or less, vias of 100 nm
diameter or less, and the like, it is necessary to detect defects
in shape, electric defect, and debris of 100 nm or less at high
speeds.
[0021] In the SEM based inspection apparatus described above, the
aforementioned inspection speed is considered to be substantially a
limit, so that a new scheme is required in order to further
increase the speed, i.e., the throughput. To satisfy this
requirement, an electron beam apparatus has been proposed for
irradiating a sample with an electron beam having a rectangular
cross section, and enlarging and detecting secondary electrons
emitted from the sample using a projection optical system (see, for
example, JP-A-2002-216694). Also, another known electron microscope
comprises a multi-pole lens to correct an axially symmetric lens
for axial chromatic aberration (see, for example, D. Ioanoviciv, et
al., Rev. Sci. Instrum., Vol. 75, No. 11, November 2004).
[0022] However, in a conventionally known projection electron beam
apparatus, as a primary beam having a large current is fed, a
resulting image is largely blurred due to the space charge effect
between electrons, thus failing to achieve a high resolution.
Accordingly, it is another object of the present invention to
provide an electron beam apparatus which is capable of preventing a
lower resolution caused by the space charge effect, and a device
manufacturing method using the apparatus.
[0023] Also, it is another object of the present invention to
provide an electron beam apparatus which is capable of evaluating a
sample without reducing the throughput even if a pattern under
evaluation is finer, and a device manufacturing method using the
apparatus.
Means for Solving the Problem
[0024] To achieve the various objects mentioned above, the present
invention provides an electron beam apparatus having a projection
electro-optical system for inspecting a surface of a sample. The
electron beam apparatus comprises:
[0025] an electron gun for emitting an electron beam;
[0026] a primary electro-optical system for guiding the emitted
electron beam onto a sample for irradiation;
[0027] a detector for detecting electrons;
[0028] a secondary electro-optical system for guiding an electron
beam bearing information on the surface of the sample, emitted from
the sample irradiated with the electron beam, to the detector,
[0029] wherein at least one of the primary electro-optical system
and the secondary electro-optical system includes a multi-pole
lens.
[0030] In the electron beam apparatus according to the present
invention, the multi-pole lens is preferably disposed between a
magnification lens of the secondary electro-optical system and beam
separating means for separating a primary electron beam and a
secondary electron beam. Preferably, in this event, an objective
lens closest to the surface of the sample comprises an
electromagnetic lens having a gap defined on a sample side, and
axial chromatic aberration caused by the electromagnetic lens can
corrected by a multi-pole lens disposed in the secondary
electro-optical system.
[0031] Also, the multi-pole lens is preferably disposed between a
reducing lens in the primary electro-optical system and beam
separating means for separating a primary electron beam and a
secondary electron beam. Preferably, in this event, the primary
electro-optical system includes an axially symmetric lens, wherein
the axially symmetric lens is set such that off-axis aberration at
an end of a visual field is equal to or less than a previously set
predetermined value, and comprises an electromagnetic lens, and the
lens has a Bohr radius larger by a factor of 50 or more than a
maximum diameter of the visual field.
[0032] Further preferably, the primary electro-optical system
comprises means for converting an electron beam from the electron
gun into multiple electron beams, and the detector comprises a
detection unit for individually detecting multiple electron beams
which make up a secondary electron beam emitted from a point on the
sample irradiated with the multiple electron beams. Furthermore,
the multi-pole lens preferably comprises quadrupole lenses at four
stages.
[0033] The present invention also provides a defect inspection
system which comprises the electron beam apparatus having a
projection electro-optical system, for inspecting a surface of a
sample for defects. The defect inspection system comprises image
capturing means for generating an image of the surface of the
sample based on information on the surface of the sample included
in electrons detected by the detector of the electron beam
apparatus, and defect evaluating means for testing the presence or
absence of a defect on the surface of the sample by comparing the
captured image with a reference image.
[0034] In the defect inspection system according to the present
invention, the system preferably further comprises a sample
transfer system for transferring the sample, a sample carrier unit
for carrying the sample thereon, an XY stage for two-dimensionally
moving the sample carrier unit, a main chamber for containing the
sample carrier unit and the XY stage, and holding the same in a
vacuum state, and a load lock chamber located between the main
chamber and the sample transfer system for holding the vacuum state
of the main chamber when the sample is moved from the sample
transfer system to the main chamber. In this event, the sample
transfer system preferably comprises an electrostatic chuck having
a function of preventing particles from sticking to the sample. The
XY stage preferably comprises an air bearing having a differential
exhaust mechanism at least in one axial direction thereof.
[0035] The electron beam apparatus having a projection
electro-optical system according to the present invention comprises
a lens system at two or more stage configured as described above
for reduction, so that a reduction ratio of 1/100-1/2000 can be
achieved while the optical path length is kept short, thus making
it possible to reduce the space charge effect.
[0036] Further, since axial chromatic aberration is corrected using
a multi-pole lens, the aperture angle can be increased, thus making
it possible to provide electron beams with a large beam current,
while the beam diameter is kept small, and consequently improve the
throughput.
[0037] Also, as described above, since the electron beam apparatus
can be improved in throughput, the defect inspection apparatus
comprising the electron beam apparatus can also improve the
throughput of inspections in a similar manner.
[0038] The present invention also found the following aspects and
solves the aforementioned problems in the following manner.
Specifically, in an electron beam apparatus, wherein an electron
beam emitted from an electron gun is shaped into a rectangular
shape through an aperture, the electron beam is caused to pass
along a trajectory deviated from secondary electrons or reflected
electrons emitted from a sample below an objective lens by the
action of a deflector, secondary electrons or reflected electrons
emitted from the sample are directed into a projection optical lens
system, an enlarge view is produced by the lens system, and a
sample image is captured by detecting the same using a TD1 or a CCD
detector to capture a sample image, the objective lens comprises a
magnetic lens, and the distance between the sample and a main
surface of the objective lens is set to be larger than a Bohr
radius of the objective lens, thereby making it possible to reduce
axial chromatic aberration, cause the electron beam to pass along a
trajectory deviated from the secondary electrons or reflected
electrons emitted from the sample below the ExB separator, avoid a
discharge in this event, and permit secondary electrons or
reflected electrons emitted from the sample in the normal direction
to intersect with the optical axis, and pass through the NA
aperture.
[0039] In the electron beam apparatus described above, the
objective lens comprises a magnetic lens, a magnetic gap is defined
on the optical axis side, and the Bohr radius of the objective lens
is set larger than the diameter of a visual field by a factor of 80
or more, thereby making it possible to reduce aberration, cause the
electron beam to pass along a trajectory deviated from the
secondary electrons or reflected electrons emitted from the sample
below the ExB separator, and focus on the surface of the sample by
the objective lens, and permit secondary electrons or reflected
electrons emitted from the sample in the normal direction to
intersect with the optical axis, and pass through the NA
aperture.
[0040] In the electron beam apparatus described above, in regard to
the magnetic lens, an axially symmetric cylinder electrode is
provided near a magnetic gap of the magnetic lens to apply a
positive high voltage, and a sufficient distance is ensured between
the cylindrical electrode and the sample to avoid a discharge,
thereby making it possible to reduce axial chromatic aberration,
cause the electron beam to pass along a trajectory deviated from
the secondary electrons or reflected electrons emitted from the
sample below the ExB separator, without giving rise to a discharge
between the sample and discharge lens, and permit secondary
electrons or reflected electrons emitted from the sample in the
normal direction to intersect with the optical axis, and pass
through the NA aperture.
[0041] In the electron beam apparatus described above, the
secondary electrons or reflected electrons emitted from the sample
are deflected by an ExB separator, and subsequently impinge on the
projection optical lens system, wherein the lens system comprises
at least one stage of an electromagnetic lens having an NA aperture
near a main surface of the lens, thereby improving the
transmittance of the secondary electrons or reflected electrons,
producing an enlarged image, and capturing two-dimensional image
data which is converted to an electric signal.
[0042] In the electron beam apparatus described above, the lens
system comprises at least one stage of an electromagnetic lens
having an NA aperture near a main surface of the lens, and the
projection optical lens system comprises an auxiliary electrostatic
lens and a magnification electromagnetic lens, wherein the
electrostatic lens comprises two or more electrodes which can be
applied with a voltage, the lens system is configured to focus a
sample image position created by a preceding lens on a main surface
of the auxiliary lens, and an electrode is selected for driving the
electrostatic lens, thereby reducing aberration, improving the
transmittance of the secondary electrons or reflected electrons,
making the magnification variable, and capture two-dimensional
image data which is converted to an electric signal.
[0043] In the electron beam apparatus described above, a lens at a
final stage comprises an electrostatic lens having at least five
electrodes, and a voltage applied to a central electrode thereof is
different in sign from voltages applied to a preceding and a
subsequent electrode thereof, thereby making it possible to
maximize the magnification, reduce the optical path length at the
same magnification, and implement this under a low aberration
condition.
[0044] In the electron beam apparatus described above, the
objective lens comprises a magnetic lens which has a Bohr diameter
on a sample side smaller than a Bohr diameter on a detection side,
thereby making it possible to cause the secondary electrons or
reflected electrons emitted in the normal direction from the
surface of the sample to pass through the NA aperture, and improve
the transmittance of the secondary electrons or reflected
electrons.
[0045] In the electron beam apparatus described above, the
objective lens comprises a magnetic lens, and electromagnetic coils
are provided at two stages before and after a main surface of the
objective lens, and these deflectors are configured to
substantially satisfy a MOL condition, thereby making it possible
to expand the visual field with the axial chromatic aberration
being kept small.
[0046] In the electron beam apparatus described above, the electron
beam apparatus is adjusted to correct field curvature aberration
and anastigmatic and reduce a difference in beam resolution between
a central area of the visual field and a peripheral area of the
visual field, thereby making it possible to reduce aberration over
the entire visual field. Further, a ball lens can be used to narrow
down a light emission direction to 1/n without causing spherical
aberration, anastigmatic, or chromatic aberration in the optical
axis direction. Also, the optical lens system can be simplified by
correcting the ball lens for chromatic aberration in radial
directions and distortions by a subsequent optical lens.
[0047] A method of manufacturing a semiconductor device comprises
preparing a wafer, preparing a mask substrate and manufacturing a
mask, performing a wafer processing step for performing required
machining to the wafer, evaluating the resulting wafer using the
electron beam apparatus according to any of claims 12 to 20, and
repeating the wafer processing step and evaluating steps a required
number of times, and cutting the wafer and assembling devices.
Here, an improved yield rate of the manufacturing can be expected
by using an electron beam apparatus which is capable of accurately
evaluating a wafer at a high throughput.
[0048] Specifically, according to the present invention, the
following inventions are provided.
[0049] (1) An electron beam apparatus for producing an enlarge view
from secondary electrons or reflected electrons emitted from a
sample using a projection optical lens system, and detecting the
enlarged view using a TD1 or a CCD detector to capture a sample
image, characterized by shaping an electron beam emitted from an
electron gun into a rectangular shape through an aperture, causing
the electron beam to pass along a trajectory deviated from the
secondary electrons or reflected electrons emitted from the sample
below an objective lens, the objective lens comprising a magnetic
lens, and setting the distance between the sample and a main
surface of the objective lens to be larger than a Bohr radius of
the objective lens.
[0050] (2) The electron beam apparatus described above is
characterized in that the Bohr radius of the objective lens is
larger than the diameter of a visual field by a factor of 80 or
more.
[0051] The electron beam apparatus characterized in that, in regard
to the magnetic lens, an axially symmetric cylinder electrode is
provided near a magnetic gap of the magnetic lens to apply a
positive high voltage, and a sufficient distance is ensured between
the cylindrical electrode and the sample to avoid a discharge
therebetween.
[0052] (4) The electron beam apparatus described above is
characterized in that the secondary electrons or reflected
electrons emitted from the sample are deflected by an ExB
separator, and subsequently impinge on the projection optical lens
system, wherein the lens system comprises at least one stage of an
electromagnetic lens having an NA aperture near a main surface of
the lens.
[0053] The electron beam apparatus, wherein the projection optical
lens system comprises an auxiliary electrostatic lens and a
magnification electromagnetic lens, the electrostatic lens
comprises two or more electrodes which can be applied with a
voltage, the lens system is configured to focus a sample image
position created by a preceding lens on a main surface of the
auxiliary lens, and the magnification is variable by selecting an
electrode for driving the electrostatic lens.
[0054] (6) In an electron beam apparatus for producing an enlarge
view from secondary electrons or reflected electrons emitted from a
sample using a projection optical lens system, and detecting the
enlarged view using a TD1 or a CCD detector to capture a sample
image, the electron beam apparatus is characterized by shaping an
electron beam emitted from an electron gun into a rectangular shape
through an aperture, causing the electron beam to pass along a
trajectory deviated from the secondary electrons or reflected
electrons emitted from the sample below an objective lens, a lens
at a final stage comprises an electrostatic lens having at least
five electrodes, and a voltage applied to a central electrode
thereof is different in sign from voltages applied to a preceding
and a subsequent electrode thereof.
[0055] The electron beam apparatus described above is characterized
in that the objective lens comprises a magnetic lens which has a
Bohr diameter on a sample side smaller than a Bohr diameter on a
detection side.
[0056] (8) The electron beam apparatus described above is
characterized in that the objective lens comprises a magnetic lens,
and electromagnetic coils are provided at two stages before and
after a main surface of the objective lens, and these deflectors
are configured to substantially satisfy a MOL condition.
[0057] (9) The electron beam apparatus described above is
characterized in that the electron beam apparatus is adjusted to
correct field curvature aberration and anastigmatic and reduce a
difference in beam resolution between a central area of the visual
field and a peripheral area of the visual field.
[0058] (10) A method of manufacturing a semiconductor device,
characterized by comprising (a) preparing a wafer, (b) preparing a
mask substrate and manufacturing a mask, (c) performing a wafer
processing step for performing required machining to the wafer, (d)
evaluating the resulting wafer using the electron beam apparatus,
and repeating the steps (c) and (d) a required number of times, and
(e) cutting the wafer and assembling devices.
[0059] According to the present invention, the electron beam
apparatus provided thereby can reduce axial chromatic aberration
and other aberration, vary the magnification of an image of
secondary electrons or reflected electrons emitted from a sample by
a projection optical lens system, without causing a discharge
between the sample and an objective lens, and improve the
transmittance, and semiconductors can be manufactured using the
apparatus.
[0060] Also, to achieve the aforementioned objects, the present
invention provides a projection electron beam apparatus
characterized by comprising:
[0061] an electron gun;
[0062] axial chromatic aberration correcting means having a
multi-pole lens; and
[0063] an objective lens for performing a MOL operation,
[0064] wherein the electron beam apparatus irradiates a sample with
an electron beam while performing a MOL operation in a divided
visual field region.
[0065] In the electron beam apparatus described above, the electron
beam apparatus is a lithography apparatus, and the apparatus
further comprises a mask or a reticle having a pattern which should
be formed on the sample. In another embodiment, the electron beam
apparatus is a sample evaluation apparatus for evaluating a pattern
formed on the sample. Also, the electron beam apparatus preferably
comprises an objective lens, and a deflector contained in the
objective lens for generating a deflection magnetic field which is
proportional to a differentiated value with respect to an optical
axis direction of an axial magnetic field distribution of the
objective lens.
[0066] The projection electron beam apparatus of the present
invention, configured as described above, can correct chromatic
aberration, and increase NA at the same resolution at which the
chromatic aberration is corrected, so that a beam current can be
increased, and processing can be performed at a high throughput.
Particularly, since it is important for a sample evaluation
apparatus to increase the inspection speed without degrading the
resolution, the present invention can provide extremely pragmatic
advantageous effects. Also, when applied to a lithography
apparatus, since NA can be increased at the same resolution at
which chromatic aberration is corrected, the space charge effect
can be reduced to further improve the throughput.
[0067] Also, to achieve the aforementioned objects, the present
invention provides an electron beam apparatus for irradiating a
sample with a rectangular primary beam and enlarging and projecting
secondary electrons emitted from the sample by an electro-optical
system to detect the secondary electrons, characterized in
that:
[0068] the electro-optical system comprises an aperture plate
having apertures arranged in a ring shape for transforming the
secondary electrons into hollow beams.
[0069] The ring-shaped apertures preferably have a width small
enough to neglect spherical aberration. Also preferably, the
electro-optical system further comprises a correction lens for
correcting the electron beam for axial chromatic aberration.
[0070] Further, to achieve the aforementioned objects, the present
invention provides an electron beam apparatus for irradiating a
sample with a primary beam emitted from an electron gun through an
objective optical system to detect secondary electrons emitted from
the sample, characterized by comprising:
[0071] an evaluation apparatus for transforming the primary beam
into hollow beams when the primary beam passes through the
objective optical system to irradiate the sample with the hollow
beams, and detecting the secondary electrons to evaluate the
sample; and
[0072] a correction lens for correcting the primary beam or the
secondary electrons for axial chromatic aberration.
[0073] The electron gun preferably comprises a cathode which has a
ring-shaped edge. Also preferably, the electron beam apparatus
further comprises a multi-aperture plate for transforming the
primary beam into multiple beams which are irradiated to the
sample, wherein the secondary electrons are detected by a plurality
of detectors.
[0074] The electron beam apparatus is preferably used in a method
of manufacturing a device, characterized by comprising the steps
of:
[0075] a. preparing a wafer;
[0076] b. performing a wafer process;
[0077] c. evaluating the wafer after undergoing the step b;
[0078] d. repeating the steps a-c a required number of times;
and
[0079] e. cutting the wafer after the step d and assembling
devices.
[0080] Further, to achieve the aforementioned objects, the present
invention provides an electron beam apparatus for scanning a sample
using a plurality of primary beams arranged in m rows and n
columns, and detecting secondary beams emitted from the sample to
evaluate the sample, characterized by:
[0081] simultaneously scanning m*n beams in a direction inclined by
an angle equivalent to 1/m in a row direction, wherein the raster
pitch of the scanning is an integer multiple of a pixel
dimension.
[0082] The present invention also provides an electron beam
apparatus for irradiating a surface of a sample with an electron
beam having a rectangular cross-section, enlarging secondary
electron beams emitted from the sample using a projection optical
system including an NA aperture plate, and capturing an image of
the sample, characterized by:
[0083] disposing the NA aperture plate or forming an optical
conjugate plane of the NA aperture plate at a position at which
aberration is minimized. The enlarged image preferably has a square
shape.
[0084] Also, the present invention provides an electron beam
apparatus for irradiating a sample with a plurality of primary
beams, separating a plurality of secondary beams emitted from the
sample from the primary beams by a beam separator, extending the
distances between the plurality of secondary electron beams by a
magnification optical system, and directing the secondary electron
beams into a detector, characterized by comprising:
[0085] a correction lens for correcting the plurality of primary
beam for axial chromatic aberration, wherein the beam separator is
disposed between the correction lens and the sample.
[0086] Also, the present invention provides an electron beam
apparatus for forming a primary beam into a beam having a
rectangular cross-section, conversing the primary beam by an
objective lens, irradiating a sample with the primary beam,
accelerating and converging secondary electron beams emitted from
the sample by the objective lens, enlarging the secondary electron
beams by a magnification optical system including an NA aperture
plate, and detecting the secondary electron beams by a sensor,
characterized in that:
[0087] the objective lens is an electromagnetic lens; and
[0088] an optical conjugate plane of the NA aperture plate is
located at a position which is passed by the secondary electron
beams emitted about a specified direction with respect to a normal
direction of the sample.
[0089] The electron beam apparatus is preferably used in a method
of manufacturing a device, characterized by comprising the steps
of:
[0090] a. preparing a wafer;
[0091] b. performing a wafer process;
[0092] c. evaluating the wafer after undergoing the step b;
[0093] d. repeating the steps a-c a required number of times;
and
[0094] e. cutting the wafer after the step d and assembling
devices.
BRIEF DESCRIPTION OF THE DRAWINGS
[0095] [FIG. 1A]
[0096] An explanatory diagram illustrating an electro-optical
system of an electron beam apparatus according to a first
embodiment of the present invention.
[0097] [FIG. 1B]
[0098] A cross-sectional view of a Wien filter in the electron beam
apparatus illustrated in FIG. 1A.
[0099] [FIG. 2]
[0100] An explanatory diagram illustrating an electro-optical
system of an electron beam apparatus according to a second
embodiment of the present invention.
[0101] [FIG. 3]
[0102] An explanatory diagram illustrating an electro-optical
system of an electron beam apparatus according to a third
embodiment of the present invention.
[0103] [FIG. 4]
[0104] An explanatory diagram showing an optical path in the
electron beam apparatus illustrated in FIG. 3.
[0105] [FIG. 5]
[0106] A graph for describing advantageous effects resulting from
removal of axial chromatic aberration in a projection electron beam
apparatus.
[0107] [FIG. 6]
[0108] An explanatory diagram illustrating a sample defect
detection system to which the electron beam apparatus of the
present invention can be applied.
[0109] [FIG. 7]
[0110] A plan view of main components of the inspection system
illustrated in FIG. 6, taken along a line B-B in FIG. 6.
[0111] [FIG. 8]
[0112] A diagram showing the relationship between a wafer carrying
box and a loader of the inspection system illustrated in FIG.
6.
[0113] [FIG. 9]
[0114] A cross-sectional view illustrating a mini-environment
device of the test system illustrated in FIG. 6, taken along a line
C-C in FIG. 6.
[0115] [FIG. 10]
[0116] A diagram illustrating a loader housing of the inspection
system illustrated in FIG. 6, taken along a line D-D in FIG. 7.
[0117] [FIG. 11]
[0118] A diagram for describing an electrostatic chuck used in an
inspection system according to the present invention.
[0119] [FIG. 12]
[0120] A diagram for describing another example of the
electrostatic chuck used in the inspection system according to the
present invention.
[0121] [FIG. 13]
[0122] A diagram for describing a further example of the
electrostatic chuck used in the inspection system according to the
present invention.
[0123] [FIG. 14]
[0124] A diagram for describing a bridge tool used in the
inspection system according to the present invention.
[0125] [FIG. 15]
[0126] A diagram for describing another example of the bridge tool
used in the inspection system according to the present
invention.
[0127] [FIG. 16]
[0128] A diagram for describing a defect inspection procedure in an
electron beam apparatus according to the present invention.
[0129] [FIG. 17]
[0130] A diagram for describing a defect inspection procedure in an
electron beam apparatus according to the present invention.
[0131] [FIG. 18]
[0132] A diagram for describing a defect inspection procedure in an
electron beam apparatus according to the present invention.
[0133] [FIG. 19]
[0134] A diagram for describing a defect inspection procedure in an
electron beam apparatus according to the present invention.
[0135] [FIG. 20]
[0136] A diagram for describing a defect inspection procedure in an
electron beam apparatus according to the present invention.
[0137] [FIG. 21]
[0138] A diagram for describing a defect inspection procedure in an
electron beam apparatus according to the present invention.
[0139] [FIG. 22]
[0140] A diagram for describing the configuration of a control
system in an inspection system according to the present
invention.
[0141] [FIG. 23]
[0142] A diagram for describing the configuration of a user
interface in the inspection system according to the present
invention.
[0143] [FIG. 24]
[0144] A diagram for describing the configuration of a user
interface in an electron beam apparatus according to the present
invention.
[0145] [FIG. 25]
[0146] A diagram for describing the configuration and an operation
procedure of an elevator mechanism in a load lock chamber in FIG.
15.
[0147] [FIG. 26]
[0148] A schematic explanatory diagram of a wafer alignment control
device which can be applied to the electro-optical system of the
inspection system according to the present invention.
[0149] [FIG. 27]
[0150] A diagram for describing reasons for which a wafer need be
aligned.
[0151] [FIG. 28]
[0152] A diagram for describing how a wafer is aligned.
[0153] [FIG. 29]
[0154] A diagram illustrating a die map in a die arranged state
after the execution of wafer alignment.
[0155] [FIG. 30]
[0156] A diagram for describing an update to coordinate values in a
wafer alignment procedure.
[0157] [FIG. 31]
[0158] A diagram for describing the amount of rotation and a
Y-direction size in the wafer alignment procedure.
[0159] [FIG. 32]
[0160] A diagram for describing interpolation of a focus value in
the creation of focus recipes during the wafer alignment
procedure.
[0161] [FIG. 33]
[0162] A diagram for describing an error produced during the wafer
alignment procedure.
[0163] [FIG. 34]
[0164] A diagram for describing a basic flow of a semiconductor
device inspection procedure.
[0165] [FIG. 35]
[0166] A diagram showing how a die under inspection is set.
[0167] [FIG. 36]
[0168] A diagram for describing the setting of an area under
inspection within a die.
[0169] [FIG. 37]
[0170] A diagram for describing a semiconductor device inspection
procedure.
[0171] [FIG. 38]
[0172] A diagram for describing a semiconductor device inspection
procedure.
[0173] [FIG. 39]
[0174] A diagram illustrating exemplary scanning and an exemplary
die under inspection when there is one die under inspection.
[0175] [FIG. 40]
[0176] A diagram for describing a method of generating a reference
image in the semiconductor device inspection procedure.
[0177] [FIG. 41]
[0178] A diagram for describing an adjacent die comparison method
in the semiconductor device inspection procedure.
[0179] [FIG. 42]
[0180] A diagram for describing an adjacent die comparison method
in the semiconductor device inspection procedure.
[0181] [FIG. 43]
[0182] A diagram for describing an adjacent die comparison method
in the semiconductor device inspection procedure.
[0183] [FIG. 44]
[0184] A diagram for describing a reference die comparison method
in the semiconductor device inspection procedure.
[0185] [FIG. 45]
[0186] A diagram for describing focus mapping in the semiconductor
device inspection procedure.
[0187] [FIG. 46]
[0188] A diagram for describing focus mapping in the semiconductor
device inspection procedure.
[0189] [FIG. 47]
[0190] A diagram for describing focus mapping in the semiconductor
device inspection procedure.
[0191] [FIG. 48]
[0192] A diagram for describing focus mapping in the semiconductor
device inspection procedure.
[0193] [FIG. 49]
[0194] A diagram for describing focus mapping in the semiconductor
device inspection procedure.
[0195] [FIG. 50]
[0196] A diagram illustrating an embodiment in which an electron
beam apparatus according to the present invention is connected to a
semiconductor manufacturing line.
[0197] [FIG. 51]
[0198] A diagram illustrating a fourth embodiment of the electron
beam apparatus according to the present invention.
[0199] [FIG. 52]
[0200] A diagram illustrating a fifth embodiment of the electron
beam apparatus according to the present invention.
[0201] [FIG. 53]
[0202] A flow chart illustrating a process for manufacturing
semiconductor devices.
[0203] [FIG. 54]
[0204] A flow chart illustrating a lithography process in the
semiconductor device manufacturing process of FIG. 53.
[0205] [FIG. 55]
[0206] A diagram generally illustrating an objective lens in the
electron beam apparatus illustrated in FIG. 52.
[0207] [FIG. 56]
[0208] An explanatory diagram illustrating an electro-optical
system of a sample evaluation apparatus which is a sixth embodiment
of the electron beam apparatus according to the present
invention.
[0209] [FIG. 57]
[0210] An explanatory diagram illustrating an electro-optical
system of an electron beam drawing apparatus which is a seventh
embodiment of the electron beam apparatus according to the present
invention.
[0211] [FIG. 58]
[0212] An explanatory diagram illustrating an electro-optical
system of a sample evaluation apparatus which is an eighth
embodiment of the electron beam apparatus according to the present
invention.
[0213] [FIG. 59]
[0214] An explanatory diagram illustrating an electron beam
scanning method in the sample evaluation apparatus illustrated in
FIG. 58.
[0215] [FIG. 60]
[0216] An explanatory diagram illustrating an electro-optical
system of a sample evaluation apparatus which is a ninth embodiment
of the electron beam apparatus according to the present
invention.
[0217] [FIG. 61]
[0218] An explanatory diagram illustrating an electro-optical
system of a transfer apparatus which is a tenth embodiment of the
electron beam apparatus according to the present invention.
[0219] [FIG. 62]
[0220] (A) and (B) are a plan view and a cross-sectional view of a
chromatic aberration corrector which has a plurality of optical
axes, which can be employed in a multi-optical-axis sample
evaluation apparatus.
[0221] [FIG. 63]
[0222] An explanatory diagram illustrating an electro-optical
system of a sample evaluation apparatus which is an eleventh
embodiment of the electron beam apparatus according to the present
invention.
[0223] [FIG. 64]
[0224] An explanatory diagram illustrating electro-optical system
of a sample evaluation apparatus which is a twelfth embodiment of
the electron beam apparatus according to the present invention.
[0225] [FIG. 65]
[0226] (A) is a diagram generally illustrating the configuration of
a thirteenth embodiment of the electron beam apparatus according to
the present invention, and (B) is a plan view of an NA aperture
plate in (A).
[0227] [FIG. 66]
[0228] (A) a diagram generally illustrating the configuration of a
fourteenth embodiment of the electron beam apparatus according to
the present invention, and (B) is a diagram for describing the
configuration of four axial chromatic aberration correction lenses
in (B).
[0229] [FIG. 67]
[0230] A diagram generally illustrating the configuration of a
fifteenth embodiment of the electron beam apparatus according to
the present invention.
[0231] [FIG. 68]
[0232] A diagram generally illustrating the configuration of a
sixteenth embodiment of the electron beam apparatus according to
the present invention.
[0233] [FIG. 69]
[0234] A beam arrangement diagram indicating a position which is
irradiated with each beam on the surface of a sample in the
electron beam apparatus of FIG. 68.
[0235] [FIG. 70]
[0236] A diagram generally illustrating the configuration of a
seventeenth embodiment of the electron beam apparatus according to
the present invention.
[0237] [FIG. 71]
[0238] A diagram illustrating an exemplary configuration of an
axial chromatic aberration correction lens in FIG. 70.
BEST MODE FOR CARRYING OUT THE INVENTION
[0239] FIG. 1A is an explanatory diagram illustrating an
electro-optical system of an electron beam apparatus which
comprises a projection electro-optical system according to a first
embodiment of the present invention. As illustrated in FIG. 1A, In
this electron beam apparatus, electron beams emitted from an
electron gun 1 are formed into a rectangular shape through a
condenser lens 2, a forming lens 4, and an aperture plate formed
with multiple apertures, and the resulting electron beams are
deflected by an ExB separator 5 so as to be perpendicular to the
surface of a sample 7, and focused on the surface of the sample 7
by an objective lens 6, such that the surface of the sample is
scanned by each electron beam in a rectangular shape. On the other
hand, secondary electrons emitted from the surface of the sample 7
by this irradiation create an enlarged image at a position 18 by
the objective lens 6, and form an enlarged image at a focal point
13 in front of a magnification lens 10 by a Wien filter 8 including
a multi-pole lens.
[0240] In this event, axial chromatic aberration can be eliminated
at the focal point 13 by the objective lens 6 and Wien filter 8
without changing the position of the focal point 13. Specifically,
the objective lens 6, which is an axially symmetric lens, generates
positive axial chromatic aberration, while the Wien filter 8
generates negative axial chromatic aberration, where the absolute
values of the positive and negative axial chromatic aberration can
be made equal by adjusting the positions of these lenses and
voltages applied to electrodes. In this way, the positive axial
chromatic aberration generated by the objective lens 6 can be
canceled out by the negative axial chromatic aberration generated
by the Wien filter 8. In this regard, the axial chromatic
aberration at the focal point 13 may be adjusted to be a negative
value near zero, thereby making it possible to cancel small
positive axial chromatic aberration generated by the magnification
lenses 10 and 11.
[0241] An enlarged image focused at the focal point 13 is further
passed through the magnification lens 10 to generate an enlarged
image in front of the magnification lens 11. Then, the enlarged
image is focused on a detection plane of a detector 12 by the
magnification lens 11. The detector 12 generates an optical signal
corresponding to the enlarged image formed thereon, and propagates
the optical signal to a CMOS image sensor (not shown) of 8.times.8,
12.times.12 or the like through an optical fiber (not shown). The
CMOS image sensor transduces the optical signal to an electric
signal which is then processed by an image data processing unit
(not shown).
[0242] While axial chromatic aberration accounts for a majority of
all aberration in an optical system, the electron beam apparatus
illustrated in FIG. 1 can adjust the enlarged image formed at the
focal point 13 such that the axial chromatic aberration is
substantially zero or a slight negative value, thus making it
possible to improve the transmittance of secondary electrons
because the aperture angle can be increased while restraining the
aberration to a certain value or less.
[0243] The electron beam apparatus illustrated in FIG. 1A is
configured not only to eliminate or reduce the axial chromatic
aberration but also to reduce off-axis aberration. Specifically,
the objective lens 6 is composed of a magnetic lens 14 having a
magnetic gap close to a sample 7-1, and an axially symmetric lens
15, as illustrated in FIG. 1A, and a Bore diameter D1 of the
magnetic lens 14 is set to be larger than the visual field diameter
by a factor of 50 or more. Also, the off-axis aberration can be
reduced by setting the distance D2 between the main surface of the
objective lens 6 and the surface of the sample 7 to be 10 mm or
more.
[0244] It should be noted that in the electron beam apparatus
illustrated in FIG. 1A, alignment deflectors may be provided at two
or more stages between the forming lens 4 and ExB separator 5,
which form part of a primary electro-optical system.
[0245] FIG. 1B illustrates a cross-sectional view of the Wien
filter 8. In this Wien filter, 12 electrodes 8-1-1-8-1-12 are
arranged around an optical axis 8-1-15, and can be applied with
independent voltages from a power supply, respectively. Reference
numeral 8-1-16 designates an insulating spacer for independently
supplying voltages to the respective electrodes. Then, by applying
currents to coils 8-1-13 and 8-1-14, it is possible to generate a
magnetic field which satisfies the Wien condition. Electron beams
originating from an image point 18-1 of a secondary electro-optical
system is focused at the center 19-1 of the Wien filter 8-1, and
again focused at a position 13-1 outside the Wien filter 8 by
appropriately setting the magnetic field and the voltages applied
to the electrodes. By thus focusing the electron beams twice, the
axial chromatic aberration presents a negative value with small
tertiary aberration over the visual field. In this regard, a
description is found in D. Ionovicin et. al, Rev. Sci. Instrum,
Vol. 75, No. 11, November, 2004.
[0246] FIG. 2 illustrates an electro-optical system of an electron
beam apparatus which comprises a projection secondary electron
detection system according to a second embodiment of the present
invention. In this electron beam apparatus, electron beams emitted
from electron gun 21 are converged by a condenser lens 23, and
irradiated to an aperture plate which has multiple apertures.
Multiple beams separated and formed by aperture plate 25 form a
first reduced image through a forming lens 26 and a reducing lens
28, and further reduced by an objective lens 34 to from a reduced
image on a sample 35. In this event, quadrupole lenses 30-1-1,
30-1-2, 31-1-1, 31-12 are provided at four stages between the
position at which the first reduced image is formed and the
objective lens 34, where these quadrupole lenses can correct axial
chromatic aberration caused by the objective lens 34.
[0247] Electron beams by secondary electrons emitted from the
sample 35 form a first enlarged image in front of a magnification
lens 36, and is further enlarged by the magnification lens 36 too
form an image which is approximately 100 times larger than the
image on the sample 35 on a detection plane of a detector 38. The
detector 38 generates an optical signal corresponding to the
enlarged image formed thereon, and propagates the optical signal to
PMT (not shown) through an optical fiber. The PMT transduces the
optical signal to an electric signal which is then processed by an
image data processing unit (not shown).
[0248] In the electron beam apparatus illustrated in FIG. 2, a Wien
filter including a multi-pole lens can be provided in the primary
electro-optical system instead of the quadrupole lens.
[0249] FIG. 3 illustrates an electro-optical system of an electron
beam apparatus according to a third embodiment of the present
invention. In this electron beam apparatus, electron beams emitted
from an electron gun 41 form a reduced image at a position 54
through an axially symmetric lens 40 which is an electromagnetic
lens, and electron beams diverged from the position 54 is focused
at a position 43 by a Wien filter 42 which converges twice.
Further, electron beams diverged from the position 43 are made to
be electron beams parallel with the optical axis by an axially
symmetric lens 44. In this embodiment, the axially symmetric lens
44 is employed at one stage, but the axially symmetric lenses may
be at two or more stages. The electron beams parallel with the
optical axis are converged onto a sample 46 through a deflector 47,
a beam separator 48 including an electromagnetic deflector, and
multi-aperture lens 45 as a plurality of electron beams.
[0250] It should be noted that since the electron gun has a
cross-over, the size of which is approximately 50 .mu.m.phi., a
cross-over image of 25 nm.phi., must be created for generating
electron beams of 50 nm.phi., and therefore the diameter .phi. of
the electron beams must be reduced approximately by a factor of
2000. The scaling factor of 1/2000 can be accomplished by reducing
the diameter approximately by a factor of 38 by the axially
symmetric lens 40 in front of the position 54, and further reducing
the diameter approximately by a factor of 60 by the multi-aperture
lens 45 at the final stage, so that multiple electron beams of 50
nm.phi. can be generated including aberration. Also, when a
multi-aperture plate 53 is provided behind the axially symmetric
lens 44, an aperture angle is reduced to reduce the axial chromatic
aberration, with the result that multiple electron beams of 50
nm.phi. can be generated with a scaling factor of approximately
1/1800. In this event, since the optical path length can be
reduced, it is possible to reduce blurs due to the space charge
effect.
[0251] In addition, the electron beam apparatus of the third
embodiment comprises the Wien filter 42 for correcting the axial
chromatic aberration between the two axially symmetric lenses 40,
44, as described above. The Wien filter 42 has a configuration
similar to the configuration described with reference to FIG. 1B,
and the multi-aperture plate 53 can be increased in diameter by
correcting the axial chromatic aberration by the Wien filter, so
that multiple electron beams can be provided with a large current
value. Quadrupole lenses at four stages may be provided instead of
the Wien filter.
[0252] The multi-aperture lens 45 is made of three metal plates
stacked one on another, and is formed with apertures in m
rows.times.n columns extending through these metal plates, as
illustrated in FIG. 4. The central metal plate is applied with a
positive high voltage. In the aperture pattern illustrated in FIG.
4, a simulation has shown that the apertures are not affected by
their adjacent apertures when the ratio d1/d2 is set to 2/3-4/5,
where d1 is the diameter of the apertures, and d2 is the pitch of
the apertures (center-to-center distance between two adjacent
apertures).
[0253] Here, the optical axis from the electron gun 41 to the
deflector 47 is intentionally offset from the optical axis of the
multi-aperture lens 45, such that they are aligned to the position
of the electromagnetic beam separator 48 by the deflector 47. The
amount of offset between the two optical axes (distance between the
two optical axes) is set to satisfy the relationship:
Amount of Offset=D3tan.sup.-1 15.degree.
where D3 is the distance between the center of deflection of the
deflector 47 and the center of deflection of the electromagnetic
beam separator 48. In this example, the amounts of deflection are
set by the beam separator 48 comprised of an electromagnetic
deflector such that the primary electron beams are deflected by
15.degree., and the secondary electrons by 16.degree., but may be
set to values other than them.
[0254] Since the secondary electrons are deflected by the beam
separator 48 comprised of an electromagnetic deflector in a
direction opposite to the direction in which the primary electrons
are deflected, the secondary electrons propagate in a direction
opposite to the direction of the primary electron beams with
respect to the right direction, i.e., the optical axis of the lens
45 in the figure, so that the second electrons are separated from
the primary electron beams. Since the secondary electron beams form
a sample image near the deflection main surface 50 (FIG. 4) of the
deflector 47, small deflection chromatic aberration affects on the
secondary electrons. The secondary electron beams separated from
the primary electron beams by the beam separator 48 form an
enlarged image on an area detector 51 through a magnification lens
49, and detected. Then, an image of the surface of the sample is
synthesized by an image data processing unit (not shown) with
reference to information on spots on the sample 46 irradiated with
the multiple primary electron beams. When the magnification is not
sufficient, magnification lenses may be provided at a larger number
of stages in the secondary optical system.
[0255] The electron beam apparatus illustrated in FIG. 3 will be
described in greater detail with reference to FIG. 4 which shows
the optical path in the apparatus. Electron beams diverged from the
cross-over formed by the electron gun 41 are narrowed down and
increased in divergence angle by the axially symmetric lens 40
which has a short focal distance, and are directed into the Wien
filter 42 and form a cross-over at the center 43 of the Wien filter
42. After exiting the Wien filter 42, the electron beams form a
cross-over at a position 56, impinges on the axially symmetric lens
44 which transforms the electron beams into collimated beams
parallel with the optical axis.
[0256] The collimated beams have their aperture angle controlled by
multiple apertures of the multi-aperture plate 53, so that the
axial chromatic aberration is kept small. Then, the collimated
beams are deflected by 15.degree. by the electromagnetic deflector
47, and deflected by 15.degree. in the opposite direction by the
beam separator 48 comprised of an electromagnetic deflector. This
is performed by supplying these two deflectors with deflection
signals which have the same absolute value but are opposite in
deflection direction. In this way, the multiple electron beams are
again perpendicular to the z-axis direction, i.e., to the surface
of the sample 46. Next, the multiple electron beams are irradiated
to the sample 46 through the multi-aperture lens 45, but no
deflection chromatic aberration is generated because the deflectors
47, 48 deflect the electron beams by the same angle in the opposite
directions to each other. Further, if an alignment signal is
multiplexed on the deflector 47 or 48, the alignment of the
multi-aperture lens 45 can be maintained to the multiple apertures
of the multi-aperture plate 53.
[0257] Secondary electrons emitted from the sample 46 are converged
into a small beam bunch by an acceleration electric field applied
thereto, and pass through the objective lens 45 without
substantially causing any loss. Then, the secondary electrons are
deflected toward the secondary optical system by the beam separator
48 which is an electromagnetic deflector to impinge on the
magnification lens 49. In FIG. 4, while the trajectory of secondary
electron beams corresponding to one lens at the center of
multi-aperture lens 45, among a plurality of secondary electron
beams from the sample 46, is represented by a dotted line, but the
secondary electron beams on the trajectory focuses at a position 50
after they have passed through the beam separator 48. Then, they
form an enlarged image on the detection plane of the detector 51 by
the action of the magnification lens 49. The detector 51 generates
an optical signal corresponding to the enlarged image formed
thereon, and propagates the optical signal to PMT (not shown) of
8.times.8, 12.times.12 or the like through an optical fiber. The
PMT transduces the optical signal to an electric signal which is
then processed by an image data processing unit (not shown).
[0258] A voltage applied to each electrode of the Wien filter and a
current applied to a coil of the same may be set with the aid of a
simulation such that the axial chromatic aberration is eliminated
in consideration of the position 54 of the cross-over, the axial
position of the Wien filter 42, the magnitude of the axial
chromatic aberration to be corrected, beam energy and the like.
[0259] In the electron beam apparatuses of the first to third
embodiments described above, the quadrupole lenses may be composed
of multi-pole lenses other than the quadrupole ones. Particularly,
quadrupole to twelve-pole lenses are preferably used, and when they
are configured in four stages, this is optimal because the former
half of the beam trajectory can be made point symmetric to the
latter half of the same to prevent secondary aberration.
[0260] Now, a description will be given of advantageous effects
resulting from the elimination of the axial chromatic aberration
using multi-pole lenses at a plurality of stages in an electron
beam apparatus which comprises a projection electro-optical
system.
[0261] In the projection electron beam apparatus, a blur ac due to
the Coulomb effect can be expressed in the following manner:
.sigma.c=IL(.alpha.V.sup.3/2)
where I: Electron Beam Current;
[0262] L: Optical Path Length;
[0263] .alpha.: aperture angle; and
[0264] V: electron beam energy.
[0265] FIG. 5 shows the result of simulation which was made n the
relationship between aberration (nm) on the surface of a sample and
the aperture angle .alpha.. A lens used in this simulation is a
tablet type which employs two stages of electrostatic lenses and
eliminates chromatic aberration of magnification. Also, a voltage
applied to the lens was set such that the electric field strength
is approximately 1.5 kV/mm on the surface of the sample. According
to the result of the simulation shown in FIG. 5, the aperture angle
a can be improved approximately by a factor of three when axial
chromatic aberration is removed. In an electron beam apparatus
which comprises a projection electro-optical system, axial
chromatic aberration is predominant among aberration on the surface
of the sample, and when the aberration on the surface of the sample
is assumed to be constant, the aperture angle a can be improved
approximately by a factor of three without removing other
aberration, if the axial color aberration is removed.
[0266] In the aforementioned equation, assuming that the blur
.delta.c is constant, the beam current I can also be increased by a
factor of three when the aperture angle .alpha. is increased by a
factor of three, and the transmittance is increased by a factor of
nine because it is a square of the aperture angle. Accordingly,
since the beam current is three times larger and the transmittance
is nine times larger, the throughput, i.e., inspection speed, which
is represented by a product of them, can be improved by a factor of
27. The inspection speed can be further improved by employing
LaB.sub.6 for an electron gun to provide a multi-beam electron beam
apparatus as in the second embodiment. A largely improved
inspection speed is extremely important in the electron beam
apparatus which comprises a projection electro-optical system.
[0267] In this regard, since SEM (scanning electron microscope)
includes a scanning deflector disposed behind a multi-pole lens,
electron beams pass only on the optical axis in the multi-pole lens
disposed in front of the deflector irrespective of deflection
scanning of the deflector. Accordingly, the axial chromatic
aberration can be readily corrected by the multi-pole lens. On the
other hand, in the electron beam apparatus which comprises a
projection electro-optical system, aberration must be reduced not
only for an image on the optical axis but also for an image spaced
apart from the optical axis, so that it has been thought that the
axial chromatic aberration is hard to correct only by simply using
a multi-pole lens. Also, no recognition has been gained that a
majority of aberration occurring in the electron beam apparatus
comprising a projection electro-optical system is axial chromatic
aberration. The inventors found through actual use tests and
simulations, which had been made using a variety of parameters,
that the axial chromatic aberration can also be favorably corrected
using a multi-pole lens even in the electron beam apparatus which
comprises a projection electro-optical system, and can thus largely
improve the throughput, as described above.
[0268] Next, a description will be given of the general
configuration of an inspection system for evaluating semiconductor
wafers, into which the electron beam apparatus of the present
invention can be incorporated for use therewith.
[0269] FIGS. 6 and 7 are a front elevation and a plan view
illustrating main components of an inspection system 61. The
inspection system 61 comprises a cassette holder 62 for holding a
cassette which stores a plurality of wafers; a mini-environment
device 63; a main housing 64; a loader housing 65 disposed between
the mini-environment device 63 and main housing 64 for defining two
loading chambers; a stage apparatus 66 disposed within the main
housing 64 for carrying a wafer W which is a wafer thereon for
transportation; a loader 67 for loading a wafer from the cassette
holder 62 onto the stage apparatus 66 disposed within the main
housing 64; and an electro-optical system 68 attached to the main
housing 64. These components are laid out in a positional
relationship as illustrated in FIGS. 6 and 7. The electron beam
apparatus of the present invention described above is incorporated
as the electro-optical system 68.
[0270] The inspection system 61 also comprises a pre-charge unit 69
disposed in the main housing 64 in vacuum; a potential application
mechanism for applying a potential to a wafer; an electron beam
calibration mechanism, and an optical microscope 206 which forms
part of an alignment controller 70 (shown in FIG. 26) for
positioning a wafer on the stage apparatus 66. The inspection
system 61 further comprises a control device CNL for controlling
operations of these components.
[0271] In the following, each of the main components (sub-systems)
of the inspection system 61 will be described in detail in regard
to the configuration.
Cassette Holder 62
[0272] The cassette holder 62 is configured to hold a plurality
(two in this embodiment) of cassettes c (for example, closed
cassettes such as SMIF, FOUP manufactured by Assist Co.) in which a
plurality (for example, twenty-five) wafers are placed side by side
in parallel, oriented in the vertical direction. The cassette
holder 62 can be arbitrarily selected for installation adapted to a
particular loading mechanism. Specifically, when a cassette is
automatically loaded into the cassette holder 62 by a robot or the
like, the cassette holder 62 having a structure adapted to the
automatic loading can be installed. When a cassette is manually
loaded into the cassette holder 62, the cassette holder 62 having
an open cassette structure can be installed. In this embodiment,
the cassette holder 62 is a type adapted to the automatic cassette
loading, and comprises, for example, an up/down table 71, and an
elevating mechanism 12 for moving the up/down table 71 up and down.
The cassette c can be automatically loaded onto the up/down table
71 in a state indicated by chain lines in FIG. 7. After the
loading, the cassette c is automatically rotated to a state
indicated by solid lines in FIG. 7 so that it is directed to the
axis of pivotal movement of a first carrier unit within the
mini-environment chamber 20. In addition, the up/down table 71 is
moved down to a state indicated by chain lines in FIG. 6.
[0273] In another embodiment, as illustrated in FIG. 8, a plurality
of 300 mm wafers W are contained in a slotted pocket (not shown)
fixed to the inner surface of a box body 73 for carriage and
storage. This wafer carrying section 74 comprises a box body 73 of
a squared cylinder, a wafer carrying in/out door 75 connected to
the box body 73 and an automatic opening apparatus for a door at a
substrate carrying in/out aperture positioned at one side of the
box body 73 and capable of mechanically opening and closing the
aperture, a lid 76 positioned in opposition to the aperture for
covering the aperture for the purpose of detachably mounting filers
and fan motors, and a slotted pocket 77 for holding a wafer W. In
this embodiment, the wafers are carried in and out by means of a
robot type carrying unit 78 of the loader 67.
[0274] It should be noted that wafers accommodated in the cassette
c are subjected to testing which is generally performed after a
process for processing the wafers or in the middle of the process
within semiconductor manufacturing processes. Specifically,
accommodated in the cassette are wafers which have undergone a
deposition process, CMP, ion implantation and so on; wafers each
formed with wiring patterns on the surface thereof; or wafers which
have not been formed with wiring patterns. Since a large number of
wafers accommodated in the cassette c are spaced from each other in
the vertical direction and arranged side by side in parallel, and
the first carrier unit has an arm which is vertically movable, a
wafer at an arbitrary position can be held by the first carrier
unit which will be described later in detail.
Mini-Environment Device 63
[0275] FIG. 9 is a front elevation illustrating the
mini-environment device 63 in a direction different to that in FIG.
7. As illustrated in FIG. 9 as well as FIGS. 6 and 7, the
mini-environment device 63 comprises a housing 80 defining a
mini-environment space 79 that is controlled for the atmosphere; a
gas circulator 81 for circulating a gas such as clean air within
the mini-environment space 79 to execute the atmosphere control; a
discharger 82 for recovering a portion of air supplied into the
mini-environment space 79 to discharge it; and a prealigner 83 for
roughly aligning a sample, i.e., a wafer placed in the
mini-environment space 79.
[0276] The housing 80 has a top wall 221, bottom wall 222, and
peripheral wall 223 which surrounds four sides of the housing 80,
to provide a structure for isolating the mini-environment space 79
from the outside. For controlling the atmosphere in the
mini-environment space 79, as illustrated in FIG. 9, the gas
circulator 23 comprises a gas supply unit 88 attached to the top
wall 85 within the mini-environment space 79 for cleaning a gas
(air in this embodiment) and delivering the cleaned gas downward
through one or more gas nozzles (not shown) in laminar flow; a
recovery duct 89 disposed on the bottom wall 96 within the
mini-environment space for recovering air which has flown down to
the bottom; and a conduit 90 for connecting the recovery duct 89 to
the gas supply unit 88 for returning recovered air to the gas
supply unit 88.
[0277] The laminar downward flow, i.e., down-flow of cleaned air is
mainly supplied such that the air passes a carrying surface formed
by the first carrier unit, later described, disposed within the
mini-environment space 79 to prevent particle particles, which
could be produced by the carrier unit, from attaching to the wafer.
An access port 91 is formed in a portion of the peripheral wall 87
of the housing 80 that is adjacent to the cassette holder 62.
[0278] A discharger 74 comprises a suction duct 92 disposed at a
position below the wafer carrying surface of the carrier unit and
below the carrier unit; a blower 93 disposed outside the housing
80; and a conduit 94 for connecting the suction duct 92 to the
blower 93. The discharger 74 aspires a gas flowing down around the
carrier unit and including particle, which could be produced by the
carrier unit, through the suction duct 95, and discharges the gas
outside the housing 84 through the conduits9 94 and the blower
93.
[0279] A prealigner 96 disposed within the mini-environment space
79 optically or mechanically detects an orientation flat (which
refers to a flat portion formed along the outer periphery of a
circular wafer and hereinafter called as orientation flat) formed
on the wafer, or one or more V-shaped notches formed on the outer
peripheral edge of the wafer, and previously aligns the position of
the wafer in a rotating direction about the axis O.sub.1-O.sub.1 at
an accuracy of approximately .+-.one degree. The prealigner 96
forms part of a mechanism for determining the coordinates of the
wafer which is a wafer, and is responsible for a rough alignment of
the wafer.
Main Housing 64
[0280] As illustrated in FIGS. 6 and 7, the main housing 64, which
defines a working chamber 97, comprises a housing body 98 that is
supported by a housing supporting device 101 carried on a vibration
blocking device, i.e., vibration isolator 100 disposed on a base
frame 99. The housing supporting device 101 comprises a frame
structure 102 assembled into a rectangular form. The housing body
98 comprises a bottom wall 103 mounted on and securely carried on
the frame structure 102; a top wall 104; and a peripheral wall 105
which is connected to the bottom wall 103 and the top wall 104 and
surrounds four sides of the housing body 32. In this embodiment,
each of the housing 98 body and the housing supporting device 101
is assembled into a rigid construction, and the vibration isolator
100 blocks vibrations from the floor, on which the base frame 99 is
installed, from being transmitted to the rigid structure. A portion
of the peripheral wall 105 of the housing 98 that adjoins the
loader housing 65 is formed with an access port 106 for introducing
and removing a wafer therethrough.
[0281] The working chamber 97 is kept in a vacuum atmosphere by a
general-purpose vacuum device (not shown). A controller 2 is
disposed below the base frame 36 for controlling the operation of
the overall inspection system 61.
[0282] In the inspection system 61, a variety of housings including
the main housing 64 are kept in vacuum atmosphere. A system for
evacuating such a housing comprises a vacuum pump, vacuum valve,
vacuum gauge, and vacuum pipes, and evaporates the housing such as
an electro-optical system portion, detector portion, wafer housing,
load lock housing or the like, in accordance with a predetermined
sequence. The vacuum valves are adjusted to kept a required vacuum
level of the housings. Further, the vacuum levels are always
monitored, and when an abnormal vacuum level is detected, an
interlock function enables isolation valves to shut dawn the path
between chambers or between a chamber and a pumping system to kept
the required vacuum level of the housing. As to the vacuum pump, a
turbo-molecular pump can be utilized for main evacuation, and a dry
pump of a Roots type can be utilized for rough evacuation. The
pressure at a test location (electron beam irradiated region) is
10.sup.-3 to 10.sup.-5 Pa. Preferably, pressure of 10.sup.-4 to
10.sup.-6 Pa is practical.
Loader Housing 65
[0283] FIG. 10 shows a front elevation of the loader housing 65,
viewed from the direction different to that in FIG. 6. As
illustrated in FIG. 10 as well as FIGS. 6 and 7, the loader housing
65 comprises a housing body 109 which defines a first loading
chamber 107 and a second loading chamber 108. The housing body 109
comprises a bottom wall 110; a top wall 111; a peripheral wall 1112
which surrounds four sides of the housing body 109; and a partition
wall 113 for partitioning the first loading chamber 107 and the
second loading chamber 108 to isolate the two loading chambers from
the outside. The partition wall 113 is formed with an aperture,
i.e., an access port 114 for passing a wafer W between the two
loading chambers. Also, a portion of the peripheral wall 112 that
adjoins the mini-environment device 63 and the main housing 64, is
formed with access ports 115, 116. The housing body 117 of the
loader housing 65 is carried on and supported by the frame
structure 102 of the housing supporting device 101. This prevents
the vibrations of the floor from being transmitted to the loader
housing 65 as well.
[0284] The access port 115 of the loader housing 65 is in alignment
with the access port 118 of the housing 80 of the mini-environment
device 63, and a shutter device 119 is provided for selectively
blocking a communication between the mini-environment space 79 and
the loading chamber 107. Likewise, the access port 116 of the
loader housing 65 is in alignment with the access port 106 of the
housing body 98, and a shutter device 120 is provided for
selectively blocking a communication between the loading chamber
108 and the working chamber 97 in a hermetic manner. Further, the
opening formed through the partition wall 121 is provided with a
shutter device 123 for closing the opening with the door 122 to
selectively block a communication between the first and second
loading chambers in a hermetic manner. These shutter devices 119,
120, 122 are configured to provide air-tight sealing for the
respective chambers when they are in a closed state.
[0285] Within the first loading chamber 107, a wafer rack 124 is
disposed for supporting a plurality (two in this embodiment) of
wafers spaced in the vertical direction and maintained in a
horizontal state.
[0286] The first and second loading chambers 107, 108 are
controlled for the atmosphere to be maintained in a high vacuum
state (at a vacuum degree of 10.sup.-5 to 10.sup.-6 Pa) by a vacuum
evacuator (not shown) in a conventional structure including a
vacuum pump, not shown. In this event, the first loading chamber
107 may be held in a low vacuum atmosphere as a low vacuum chamber,
while the second loading chamber 108 may be held in a high vacuum
atmosphere as a high vacuum chamber, to effectively prevent
contamination of wafers. The employment of such a loading housing
structure including two loading chambers allows a wafer W to be
carried, without significant delay from the loading chamber the
working chamber. The employment of such a loading chamber structure
provides for an improved throughput for the defect testing, and the
highest possible vacuum state around the electron source which is
required to be kept in a high vacuum state.
[0287] The first and second loading chambers 107, 108 are connected
to vacuum pumping pipes and vent pipes for an inert gas (for
example, dried pure nitrogen) (neither of which are shown),
respectively. In this way, the atmospheric state within each
loading chamber is attained by an inert gas vent (which injects an
inert gas to prevent an oxygen gas and so on other than the inert
gas from attaching on the surface).
[0288] In the main housing 64 using electron beams, when
representative lanthanum hexaboride (LaB.sub.6) used as an electron
source for an electro-optical system, later described, is once
heated to such a high temperature that causes emission of thermal
electrons, it should not be exposed to oxygen within the limits of
possibility so as not to shorten the lifetime. Since the exposure
to oxygen is made less likely by carrying out the atmosphere
control as mentioned above at a stage before introducing the wafer
W into the working chamber of the main housing 64 in which the
electro-optical system 68 is disposed, the lifetime of the electron
source is less likely to be shortened.
Stage Apparatus 66
[0289] The stage apparatus 66 comprises a fixed table 125 disposed
on the bottom wall 103 of the main housing 64; a Y-table 126
movable in a Y direction on the fixed table (the direction vertical
to the drawing sheet in FIG. 6); an X-table 127 movable in an X
direction on the Y-table 126 (in the left-to-right direction in
FIG. 6); a turntable 128 rotatable on the X-table; and a holder 129
disposed on the turntable 128. A wafer W is releasably held on a
wafer carrying surface 551 of the holder 129. The holder 129 may be
of a general-purpose structure which is capable of releasably
chucking a wafer by means of a mechanical or electrostatic chuck
feature. The stage apparatus 66 uses servo motors, encoders and a
variety of sensors (not shown) to operate the plurality of tables
126-128 mentioned above to permit highly accurate alignment of a
wafer W held on the carrying surface 130 by the holder 129 in the X
direction, Y direction and Z-direction (the Z-direction is the
up-down direction in FIG. 6) with respect to electron beams
irradiated from the electro-optical system 68, and in a direction
(.theta. direction) about the axis normal to the wafer supporting
surface.
[0290] The alignment in the Z-direction may be made such that the
position on the carrying surface of the holder 129, for example,
can be finely adjusted in the Z-direction. In this event, a
reference position on the carrying surface is sensed by a position
measuring device using a laser of an extremely small diameter (a
laser interference range finder using the principles of
interferometer) to control the position by a feedback circuit (not
shown). Additionally or alternatively, the position of a notch or
an orientation flat of a wafer is measured to sense a plane
position or a rotational position of the wafer relative to the
electron beam to control the position of the wafer by rotating the
turntable 128 by a stepping motor which can be controlled in
extremely small angular increments. It may be possible to remove
the holder 129 and carry a wafer W directly on the rotatable table
128. In order to maximally prevent particle produced within the
working chamber 97, servo motors 131, 132 and encoders 133, 134 for
the stage apparatus 66 are disposed outside the main housing
64.
[0291] It is also possible to establish a basis for signals which
are generated by previously inputting a rotational position, and
X-Y-positions of a wafer relative to the electron beams in a signal
detecting system or an image processing system, later
described.
[0292] The wafer chucking mechanism provided in the holder is
configured to apply a voltage for chucking a wafer to an electrode
of an electrostatic chuck, and the alignment is made by pinning
three points on the outer periphery of the wafer (preferably spaced
equally in the circumferential direction). The wafer chucking
mechanism comprises two fixed aligning pins and a push-type clamp
pin. The clamp pin can implement automatic chucking and automatic
releasing, and constitutes a conducting spot for applying the
voltage.
[0293] While in this embodiment, the X-table is defined as a table
which is movable in the left-to-right direction in FIG. 7; and the
Y-table as a table which is movable in the up-down direction in
FIG. 13, a table movable in the left-to-right direction may be
defined as the Y-table; and a table movable in the up-down
direction as the X-table in the same figures.
Wafer Chucking Mechanism
1) Basic Structure of Electrostatic Chuck:
[0294] For accurate and rapid focusing in the electro-optical
system, ruggedness on the surface of a sample or a wafer is
preferably as small as possible. For this reason, a wafer is
absorbed on the surface of an electrostatic chuck which is
manufactured with a high flatness (flatness of 5 .mu.m or less is
preferable).
[0295] The structure for the electrode of the electrostatic chuck
is classified into a monopole type and a dipole type. The monopole
type electrostatic chuck previously brings a wafer into conduction,
and applies a high voltage (generally approximately in a range of
several tens to several hundreds of volts) between the single
electrode of the electrostatic chuck and the wafer, to absorb the
wafer. The dipole type electrostatic chuck need not bring a wafer
into conduction, but can absorb a wafer by only applying a positive
and a negative voltage to two electrodes of the electrostatic
chuck, respectively. However, generally, for ensuring a stable
absorption condition, the two electrodes must be formed into an
interdigital shape, so that the electrodes are in a complicated
shape.
[0296] On the other hand, for testing a sample, a wafer must be
applied with a predetermined voltage (retarding voltage) in order
to establish a focusing condition for the electro-optical system,
or in order to facilitate electronic observations on the state of
the surface of a sample. The electrostatic chuck must be the
monopole type in order to apply the retarding voltage to a wafer
and to stabilize the potential on the surface of the wafer.
(However, the electrostatic chuck must be operated to be a dipole
type until the wafer is brought into conduction with a conduction
needle, as will be described later. To meet this requirement, the
electrostatic chuck is configured to be switchable between a
monopole mode and a dipole mode.) When the potential on the surface
of the wafer is not stable at a predetermined value in each test
mode, the focusing condition is not satisfied, resulting in a
failure of generating a clear image. It is therefore necessary to
securely confirm that the wafer is conducting before the
application of the retarding voltage.
[0297] Therefore, a mechanical contact with the wafer is involved
in bringing the wafer into conduction. However, increasingly strict
requirements are imposed on wafers for preventing contaminations,
and it is therefore requested that a mechanical contact to a wafer
be made with the least possible frequency, so that a contact to the
edge of a wafer may not be permitted. In this event, the wafer must
be brought into conduction through the back thereof.
[0298] A wafer is generally formed with a silicon oxide film on its
back, so that the conduction cannot be established unless the
silicon oxide film is partially removed from the back. To do this,
needles are brought into contact with the back of the wafer at two
or more locations, and a voltage is applied between the needles to
locally break the oxide film, thereby making it possible to
successfully bring the wafer into conduction. The voltage applied
between the needles may be a DC voltage or an AC voltage of
approximately several hundreds of volts. The needles are required
to be made of a refractory material which is non-magnetic and
wear-resistant, for example, tungsten. Further, for enhancing the
durability or for preventing contaminations of wafers, the needles
may be effectively coated with TiN or diamond. In addition, for
confirming that the wafer is conducting, a voltage is effectively
applied between the needles to measure a current with an ampere
meter. By applying the retarding voltage after the confirmation of
the conduction, the surface of the wafer can be charged with a
desired potential, thus conducting a test while satisfying the
focusing condition.
[0299] A chucking mechanism as illustrated in FIG. 11 has been
created from the foregoing background. An electrostatic chuck is
provided with electrodes which preferably have an interdigital
shape for stably absorbing a wafer W; three pusher pins 143 for
passing a wafer; and two or more conduction needles 144 for
applying a voltage to a wafer. In addition, a correction ring 141
and a wafer dropping mechanism 142 are disposed around the
electrostatic chuck.
[0300] The pusher pins 143 previously protrude from the surface of
the electrostatic chuck when a sample wafer W is transferred by a
robot hand, and slowly move down as the wafer W is placed on the
electrostatic chuck by the action of the robot hand to receive the
wafer W on the electrostatic chuck. When a wafer W is removed from
the electrostatic chuck, the pusher pins 143 perform the reverse
actions to pass the wafer W to the robot hand. The pusher pins 143
must be made of a material which contributes to prevention of a
shifted position and contamination of the wafer, and silicone
rubber, rubber fluoride, ceramics such as SiC, alumina or the like,
resin such as teflon, polyimide or the like, are preferably used
for the pusher pins 143.
[0301] The pusher pins 143 can be driven by several possible
methods. A first method involves the installation of a non-magnetic
actuator below the electrostatic chuck. Specifically, the pusher
pins are directly driven linearly by an ultrasonic linear motor, or
the pusher pins are linearly driven by a combination of a rotary
ultrasonic motor and a ball screw or a rack-and-pinion gear. With
this method, the pusher mechanism can be integrated in compact on
the table of the XY-stage on which the electrostatic chuck is
mounted, whereas excessively many wires are required for actuators,
limit sensors and the like. These wires run from the table on which
the XY-stage operates to a wall surface of a sample chamber (main
chamber or main housing), and bend in association with the actions
of the stage, so that the wires must be routed with large radii of
curvature R, resulting in a need for a large space. Also, the wires
can be a source of particles, and must be replaced on a periodic
basis, so they should be used in a minimally required amount.
[0302] An alternative method may supply a driving force from the
outside. As the stage is moved to a position at which a wafer W is
removed, a shaft protruding into a vacuum through a bellows is
driven by an air cylinder disposed outside the chamber to push a
shaft of a pusher driving mechanism disposed below the
electrostatic chuck. The shaft is connected to a rack-and-pinion
gear or a link mechanism within the pusher driving mechanism, such
that reciprocal movements of the shaft are associated with
up-and-down movements of the pusher pins. When a wafer W is passed
to the robot hand, the pusher pins 143 are moved up by adjusting
the speed to a proper level by a controller and pushing the shaft
out into the vacuum by the air cylinder.
[0303] The external shaft driving force is not limited to the air
cylinder, but may be implemented by a combination of a servo motor
with a rack-and-pinion gear or a ball screw. Alternatively, the
external driving source can be a rotary shaft. By this strategy,
the rotary shaft is coupled through a vacuum sealing mechanism such
as a magnetic fluid seal or the like, and the pusher driving
mechanism contains a mechanism for converting rotations into linear
motions of pusher pins.
[0304] The correction ring 141 has an action of holding a uniform
electric field distribution around the edge of a wafer, and is
basically applied with the same potential as the wafer. However,
the correction ring 141 may be applied with a potential slightly
different from the potential at the edge of the wafer in order to
cancel out the influence of a narrow gap between the wafer and the
correction ring, and of a small difference in height between the
surfaces of the wafer and the correction ring. The correction ring
141 has a width of approximately 10-30 mm in a radial direction of
the wafer, and can be made of a non-magnetic and conductive
material, for example, titanium, phosphor bronze, TiN or Tic coated
aluminum, or the like.
[0305] Each of the conduction needles 144 is supported by a spring
147, and as a wafer W is placed on the electrostatic chuck, the
conduction needles 144 are lightly urged onto the back of the wafer
by the forces of the springs. In this state, a voltage is applied
in a manner described above to bring the wafer W into electric
conduction.
[0306] The electrostatic chuck body comprises non-magnetic flat
electrodes 19-1, 19-2 made of tungsten or the like, and dielectric
films formed on the electrodes. The dielectric films may be made of
alumina, aluminum nitride, polyimide or the like. Generally,
ceramics such as alumina are perfect insulating materials having a
volume resistivity of approximately 10.sup.14 .OMEGA.cm, so that no
charge migration occurs within the material, and a Coulomb force
acts as an absorption force. However, by slightly adjusting the
composition of ceramics, the volume resistivity can be reduced to
approximately 10.sup.10 .OMEGA.cm, permitting charges to migrate
within the material to cause a so-called Jonson-Rahbeck force to
act as a wafer absorption force which is stronger than the Coulomb
force. The stronger the absorption force is, a correspondingly
lower voltage can be applied to the wafer, a larger margin can be
ensured for breakdown, and a stable absorption force is more likely
to be provided. Also, by machining the surface of the electrostatic
chuck into a dimple shape, particles or the like, even sticking to
the surface of the electrostatic chuck, are likely to drop into
valleys of dimples, leading to an expected effect of reducing the
possibility of affecting the flatness of the wafer.
[0307] Bearing the foregoing discussion in mind, the electrostatic
chuck suitable for practical use may be made of such material as
aluminum nitride or alumina ceramics which is adjusted to have the
volume resistivity of approximately 10.sup.10 .OMEGA.cm, and formed
with ruggedness such as dimples on the surface which is machined
such that a surface formed of a collection of convex portions has a
flatness of approximately 5 .mu.m.
2) Chucking Mechanism for 200/300 Bridge Tool:
[0308] The inspection apparatus may be required to test two types
of wafers of 200 mm and 300 mm diameters without mechanical
modifications. In this event, the electrostatic chuck must have the
ability to chuck the wafers in two sizes, and a correction ring
compatible with the two wafer sizes must be provided along the
periphery of the wafer. FIGS. 11(A), 11(B) and FIG. 12 illustrate
the structure for meeting the foregoing requirements.
[0309] FIG. 11(A) illustrates how a 300-mm wafer W is placed on
the-electrostatic chuck. The correction ring 19-1, which has an
inner diameter slightly larger than the size of the wafer W
(defining a gap of approximately 0.5 mm therebetween), is
positioned by and carried on a metal ring part disposed along the
outer periphery of the electrostatic chuck with a spigot joint. The
correction ring 141 is provided with wafer dropping mechanisms 142
at three locations. Each of the wafer dropping mechanisms 142 is
driven by a vertical driving mechanism associated with a mechanism
for driving the pusher pins 143, and is supported for rotation
about the rotating shaft arranged in the correction ring 141.
[0310] When a wafer W is received from a robot hand, the pusher pin
driving mechanism operates to push up the pusher pins 143. The
wafer dropping mechanisms 142 provided in the correction ring 141
also receive a driving force to rotate at a timing appropriate to
the operation of the pusher pin driving mechanism, as illustrated
in FIG. 11(B). Consequently, each wafer dropping mechanism 142
forms a tapered surface for guiding the wafer W about the
electrostatic chuck. Next, after the wafer W is placed on the
pusher pins 143 thus pushed up, the pusher pins 143 are moved down.
By adjusting a timing at which the driving force acts on the wafer
dropping mechanism 142 to be appropriate to the lowering of the
pusher pins 143, the wafer W is placed on the electrostatic chuck
such that the center of the wafer W substantially matches the
center of the electrostatic chuck while the position of the wafer W
is modified by the tapered surfaces of the wafer dropping
mechanisms 142.
[0311] Desirably, the tapered surface of each wafer dropping
mechanism 142 is coated with a low-friction material such as
Teflon, or preferably a conductive low-friction material (for
example, conductive Teflon, conductive diamond-like carbon, TIN
coating). In FIG. 11(A), terminals A, B, C, D and E are applied
with respective appropriate voltages (later described), and wafer
conduction needles 144 sense that a wafer W is placed on the
electrostatic chuck, and are pushed up by associated springs
145.
[0312] FIG. 12 illustrates a 200-mm wafer W placed on the same
electrostatic chuck. Since the surface of the electrostatic chuck
is exposed due to the diameter of the wafer smaller than that of
the electrostatic chuck, the electrostatic chuck is mounted with a
correction ring 151 which has a size large enough to completely
cover the electrostatic chuck. The positioning of the correction
ring 151 is performed in a similar manner to the correction ring
for 300-mm wafer.
[0313] The correction ring 151 is formed with a step along the
inner periphery, such that the step fits into an annular groove
151' of the electrostatic chuck. This is a structure for covering
the surface of the electrostatic chuck with a conductor (correction
ring 151), when a 200-mm wafer W is placed on the electrostatic
chuck, such that the surface of the electrostatic chuck is
invisible from the gap between the inner periphery of the
correction ring 151 and the outer periphery of the wafer W. If the
surface of the electrostatic chuck were visible, charges would be
accumulated on the surface of the electrostatic chuck when electron
beams are irradiated and would cause disturbance of the potential
on the surface of the sample.
[0314] For replacing the correction ring 151, a correction ring
replacement station has been installed at a predetermined location
within the vacuum chamber, and a correction ring of a required size
is transferred by a robot from the station, and mounted on the
electrostatic chuck (inserted into a spigot joint).
[0315] The 200-mm wafer correction ring is also provided with wafer
dropping mechanisms 20.cndot.2 similar to those of the 300-mm wafer
correction ring. The electrostatic chuck is formed with a relief
portion for preventing interference with the wafer dropping
mechanisms 20-2. A 200-mm wafer is placed on the electrostatic
chuck completely in the same manner as the 300-mm wafer. The
electrostatic chuck comprises terminals A, B, C, D, E for receiving
respective appropriate voltages, push pins 153 similar to the push
pins 143, and wafer conduction needles 154 similar to the wafer
conduction needles 144.
[0316] FIGS. 13(A) and 13(B) generally illustrate the configuration
of an electrostatic chuck which can support both of 300-mm wafer
and 200-mm wafer, wherein FIG. 13(A) illustrates a 300-mm wafer
placed on the electrostatic chuck, and FIG. 13(B) illustrates a
200-mm wafer placed on the electrostatic chuck. As is understood
from FIG. 13(A), the electrostatic chuck has an area large enough
to accommodate a 300-mm wafer, and as illustrated in FIG. 13(B), a
central portion of the electrostatic chuck has an area large enough
to accommodate a 200-mm wafer. A groove 156 is formed to surround
the central portion of the electrostatic chuck for fitting the
inner periphery of the correction ring 151. The electrostatic chuck
also comprises terminals A, B, C, D and E for receiving respective
appropriate voltages.
[0317] In the electrostatic chuck illustrated in FIGS. 13(A) and
13(B), detections may be optically made as to whether or not a
wafer is placed on the electrostatic chuck, whether or not a wafer
is correctly placed on the electrostatic chuck, whether or not the
correction ring is used, and the like. For example, an optical
sensor may be disposed above the electrostatic chuck, in which case
detection can be made as to whether a wafer is evenly placed or is
inclinedly placed by measuring the length of an optical path when
light emitted from the optical sensor is reflected back by the
wafer to return again to the optical sensor. Also, the presence or
absence of the correction ring can be detected by a light emitter
which inclinedly emits light to an appropriate point within the
area on which the correction ring should be mounted, and a light
receiver which receives reflected light from the correction ring.
Further, it is possible to detect which of the 300-mm wafer
correction ring or 200-mm wafer correction ring is mounted on the
electrostatic chuck by providing a combination of a light emitter
which inclinedly emits light to an appropriate point in the area on
which the 200-mm wafer correction ring should be mounted and a
light receiver which receives reflected light from the correction
ring, and a combination of a light emitter which inclinedly emits
light to an appropriate point in the area on which the 300-mm wafer
correction ring should be mounted and a light receiver which
receives reflected light from the correction ring, and detects
which light receiver receives the reflected light.
3) Wafer Chucking Procedure:
[0318] The wafer chucking mechanism which has the structure
described above chucks a wafer in the following procedure.
[0319] (1) A correction ring suited to a wafer size is transferred
by a robot, and mounted on the electrostatic chuck.
[0320] (2) The wafer is transferred by a robot hand, and placed on
the electrostatic chuck through vertical movements of the pusher
pins.
[0321] (3) The electrostatic chuck is applied with voltages (a
positive and a negative voltage are applied to the terminals C and
D, respectively) in the dipole mode to absorb the wafer.
[0322] (4) A predetermined voltage is applied across the conduction
needles to break the insulating film (oxide film) on the back of
the wafer.
[0323] (5) A current between the terminals A and B is measured to
confirm whether or not the wafer is conducting.
[0324] (6) The electrostatic chuck is switched to the monopole
absorption mode (a ground potential GRD is applied to the terminals
A, B, while the same voltage is applied to the terminals C and
D).
[0325] (7) The voltage at the terminal A (or B) is reduced while
maintaining a potential difference between the terminal A (or B)
and the terminal C (or D), and the wafer is applied with a
predetermined retarding voltage.
Configuration of Apparatus for 200/300 Bridge Tool
[0326] FIGS. 14 and 15 illustrate a detection system which is
capable of testing either of 200-mm wafers and 300-mm wafers
without mechanical modifications. The following description will be
centered on aspects different from inspection apparatuses dedicated
to 200-mm wafers or 300-mm wafers.
[0327] In storage spaces 21.cndot.1 for storing wafer cassettes
which are picked up in accordance with particular specifications
such as 200/300-mm wafer, FOUP, SMIF, open cassette and the like, a
variety of wafer cassettes can be stored in accordance with wafer
sizes and types of wafer cassettes determined by specifications
determined by the user. An atmosphere transfer robot 21.cndot.2 has
a hand which can support different sizes of wafers, and more
specifically, is formed with a plurality of wafer receiving steps
suited to respective wafer sizes, such that a wafer is placed on
the hand at a location appropriate to its size. The atmosphere
transfer robot 161 transfers wafers from the storage space 162 to a
pre-aligner 163 to adjust the orientation of the wafers, and then
removes the wafers from the pre-aligner 163 for delivery into a
load lock chamber 164.
[0328] A wafer rack within the load lock chamber 164, which is also
in a similar structure, has a wafer support formed with a plurality
of receiving steps suited to respective wafer sizes. The robot hand
is adjusted in height such that a group of wafers fit into the
receiving step suited to their size. The wafers placed on the hand
of the atmosphere transfer robot 161 are loaded into a wafer rack,
and then the robot hand is moved down to fit the wafers into a
predetermined receiving step of the wafer support.
[0329] Each of the wafers placed in the wafer rack within the load
lock chamber 164 is next removed from the load lock chamber 163 by
a vacuum transfer robot 166 arranged in a transfer chamber 165, and
transferred onto a stage 168 within a sample chamber 167. The
vacuum transfer robot 166 also has a hand which is formed with a
plurality of receiving steps suited to respective wafer sizes,
similar to the atmosphere transfer robot 21.cndot.2. The wafer
fitted into a predetermined receiving step of the robot hand is
placed on the electrostatic chuck which has been previously mounted
with a correction ring suited to the wafer size, and securely
absorbed by the electrostatic chuck on the stage 168. The
correction ring 169 is placed on a correction ring rack 170
disposed within the transfer chamber 165. Here, the vacuum transfer
robot 166 picks up a correction ring 169 suited to the wafer size
from the correction ring rack 170, and mounts the correction ring
onto the electrostatic chuck. After fitting the correction ring 169
into a positioning spigot joint formed on the outer periphery of
the electrostatic chuck, the wafer is placed on the electrostatic
chuck.
[0330] When a correction ring is to be replaced with another one,
operations reverse to the foregoing are performed. Specifically,
the correction ring 169 is removed from the electrostatic chuck by
the robot 166, and transferred back into the correction ring rack
170 within the transfer chamber 165. Then, a correction ring suited
to the size of a wafer which is to be tested is transferred from
the correction ring rack 170 to the electrostatic chuck.
[0331] In the inspection apparatus illustrated in FIG. 14, the
pre-aligner 163 is positioned near the load lock chamber 164, so
that even if a correction ring cannot be mounted in the load lock
chamber due to an improperly aligned wafer, the wafer can be
readily transferred back to the pre-aligner to again align the
wafer, thus advantageously reducing a time loss in the process.
[0332] FIG. 15 illustrates an exemplary inspection system in which
correction rings are stored at different places. The correction
ring rack 170 is omitted. The load lock chamber 171 is formed with
a wafer rack and a correction ring lack in a layered structure.
These racks are installed on an elevator and can therefore be moved
up and down. First, for mounting the electrostatic chuck with a
correction ring suited to the size of a wafer which is to be
tested, the elevator of the load lock chamber 171 is moved to a
position at which the vacuum transfer robot 166 can pick up the
correction ring. Then, after the correction ring has been mounted
on the electrostatic chuck by the vacuum transfer robot 166, the
elevator is operated to carry a wafer to be tested, and the wafer
is removed from the wafer rack by the vacuum transfer robot 166,
and then placed on the electrostatic chuck. This configuration,
though the elevator is required in the load lock chamber 171, can
effectively reduce the vacuum transfer chamber 165 held in volume,
and also reduce the foot print of the apparatus.
[0333] Using the algorithm as described above, an alignment of a
wafer on the stage is conducted. While a sensor for sensing whether
or not a wafer is placed on the electrostatic chuck is preferably
disposed at a position at which the sensor can support any of
different wafer sizes, a plurality of sensors which are identical
in function may be provided for respective wafer sizes if such a
position is not available.
[0334] Now, description will be made of a whole procedure for a
defect test. As illustrated in FIG. 16, the defect test involves
moving the stage while irradiating electron beams for TDI scan
imaging (FIG. 17), and using a test dedicated processing unit (IPE)
in accordance with set inspection conditions (an array inspection
condition, a random inspection condition, an area under inspection)
for inspecting a sample for defects in real time.
[0335] Inspection recipes set conditions for the electro-optical
systems, dies under inspection, area under inspection, a inspection
method (random/array), and the like (FIGS. 18(A) and 18(B)). For
capturing stable images for the defect test, the inspection
apparatus simultaneously makes an EO correction for limiting the
shaking of captured images due to shifted positions, speed
variations and the like; a die position correction for absorbing an
error between an ideal placement on a die map and an actual die
position; and a focus adjustment for compensating for a focus value
of the overall wafer area using a focus value previously measured
at a finite measuring point in real time.
[0336] In a scanning operation involved in the defect test, instead
of testing the entire area of a die under inspection (FIG. 19), an
intermittent test can also be made by adjusting step moving
increments in a scanning direction and an orthogonal direction, as
illustrated in FIG. 20 (for reducing a test time).
[0337] After completion of the test, the result of the test is
displayed on a display device, including the number of defects,
positions of the dies including defects, sizes of the defects,
positions of defects within each die, types of defects, images of
the defects, and images for comparison. If the foregoing
information, recipe information and the like are saved in a file,
the results of past tests can be confirmed and reproduced.
[0338] During an automatic defect test, a selection of a variety of
recipes triggers loading a wafer in accordance with a transfer
recipe, aligning the wafer on the stage in accordance with an
alignment recipe, setting focus conditions in accordance with a
focus map recipe, conducting a test in accordance with a test
recipe, and unloading the wafer in accordance with the transfer
recipe (FIGS. 20(A) and 20(B)).
Control Device CNL
[0339] The control device CNL (FIG. 6) for the defect test
comprises a plurality of controllers as illustrated in FIG. 22.
[0340] A main controller, which governs a GUI unit and sequence
operations of the apparatus (EBI), receives operation instructions
from a factory host computer or GUI, and gives necessary
instructions to a VME controller and an IPE controller. The main
controller is provided with a man-machine interface through which
the operator performs operations (entering a variety of
instructions/commands, recipes and the like, instructing the start
of a test, entering all necessary commands for switching between an
automatic and a manual test mode, commands involved in the manual
test mode, and the like). Otherwise, the main controller is
responsible for communications with the host computer in the
factory, control of an evacuation system, transfer of wafers,
control of positioning, transmission of commands to and reception
of information from a stage controller and other controllers, and
the like. The main controller also has a stage vibration correcting
function for capturing an image signal from an optical microscope
and feeding a stage fluctuation signal back to the electro-optical
system to correct deteriorated images, and an automatic focus
correcting function for detecting a displacement of a wafer
observation position in the Z-axis direction (axial direction of
the secondary optical system) and feeding the detected displacement
to the electro-optical system to automatically correct the focus.
The transmission and reception of feedback signals to and from the
electro-optical system, as well as the transmission and reception
of signals to and from the stage apparatus are performed through
the IPE controller and stage controller, respectively.
[0341] The VME controller governs the operation of component
devices of the apparatus (EBI), and gives instructions to the stage
controller and PLC controller in accordance with instructions from
the main controller.
[0342] The IPE controller acquires defect test information from an
IPE node computer, classifies acquired defects, and displays-images
of the defects thus classified. The IPE node computer acquires
images output from a TDI camera, and conducts a defect test. The
IPE node computer also controls the electro-optical system 68,
i.e., controls the electron gun, lenses, aligner and the like. The
IPE node computer controls automatic voltage setting and the like
for the respective lens systems and aligner corresponding to each
operation mode (associative control); for example, controlling a
power supply such that a constant electron current is irradiated to
a target area at all times even if a different scaling factor is
selected, and automatically setting voltages to the respective lens
systems and aligner corresponding to each scaling factor.
[0343] The PLC controller receives instructions from the VME
controller, drives devices such as valves, acquires sensor
information, and monitors for abnormalities such as an abnormal
degree of vacuum which must be monitored at all times.
[0344] The stage controller receives instructions from the VME
controller, and moves the stage in the X- and Y-directions as well
as rotating a wafer placed on the stage. In particular, the stage
controller enables precise movements on the order of .mu.m in the
X-axis direction and Y-axis direction (with a tolerance of
approximately .+-.0.5 .mu.m), and also enables a control in the
rotating direction (.theta. control) within an error accuracy of
approximately .+-.0.3 seconds.
[0345] With the configuration of a distributed control system as
described above, even if a component device is changed at an end,
no change is required in software and hardware of higher rank
controllers, due to maintaining the same interfaces between the
respective controllers. Also, even if a sequence operation is added
or modified, a flexible support can be provided for a change in
configuration by minimizing changes in higher rank software and
hardware.
User Interface
[0346] FIG. 23 illustrates a device configuration in the user
interface. An input section shows devices which receive entries
from the user, and comprises "keyboard," "mouse," and "JOY pad." A
display section shows devices for displaying information to the
user, and comprises two monitors. A monitor 1 displays an image
captured by a CCD camera or a TDI camera, while a monitor 2
displays a GUI screen.
[0347] In this regard, the progress of a test may be displayed on
the screen in real time using different colors. The progress of a
test will become apparent if different colors are used to display
wafer location information indicative of where a certain wafer is
found, and information on wafers under inspection such as to which
stage the test has been made, where defects are found on each
wafer, and the like. Also, dies under inspection may be displayed
every swath.
[0348] The present apparatus defines the following three coordinate
systems.
(1) Stage Coordinate System [X.sub.S, Y.sub.S]
[0349] This is the reference coordinate system for indicating a
position during stage position control, and only one stage
coordinate system exists in the apparatus.
[0350] The lower left corner of the chamber is defined to be the
origin, and the X-coordinate value increases in the right
direction, while the Y-coordinate value increases in the upward
direction.
[0351] A position (coordinate values) represented by the stage
coordinate system is the center of the stage (the center of a
wafer). In other words, when coordinate values [0, 0]are specified
in the stage coordinate system, the center of the stage (center of
a wafer) moves to match the origin of the stage coordinate
system.
[0352] The unit is [.mu.m], but a minimum resolution is defined to
be .lamda./1,024 (-0.618 [.mu.m]), where .lamda. is the wavelength
of a laser used in a laser interferometer (.lamda.632.991
[.mu.m]).
(2) Wafer Coordinate System [X.sub.W, Y.sub.W]
[0353] This is a reference coordinate system for indicating a
position on a wafer which is to be observed (imaged and displayed),
and only one wafer coordinate system exists in the apparatus.
[0354] The center of a wafer is defined to be the origin, and the
X-coordinate value increases in the right direction, while the
Y-coordinate value increases in the upward direction. A position
indicated in the wafer coordinate system (coordinate values) is the
center of imaging in an imaging device (CCD camera, TDI camera)
selected at that time.
[0355] The unit is [.mu.m], but a minimum resolution is defined to
be .lamda./1,024 (-0.618 [.mu.m]), where .lamda. is the same as the
foregoing.
(3) Die Coordinate System [X.sub.D, Y.sub.D]
[0356] This is a reference coordinate system for defining a
position on each die which is to be observed (imaged and
displayed), and exists on each die.
[0357] The lower left corner of each die is defined to be the
origin, and the X-coordinate value increases in the right
direction, while the Y-coordinate value increases in the upward
direction.
[0358] The unit is [.mu.m], but a minimum resolution is defined to
be .lamda./1,024 (-0.618 [.mu.m]), where .lamda. is the same as the
foregoing. Dies on a wafer are numbered, and a die which is the
basis for the numbering is called the "origin die." By default, the
origin die is the one closest to the origin of the wafer coordinate
system, but the position of the origin die can be selected in
response to a designation of the user.
[0359] The relationship between the coordinate values in the
respective coordinate systems and a position at which an
observation (display) is made is as shown in FIG. 24.
[0360] Also, the relationship between coordinates indicated by the
user interface and a direction in which the stage is moved is as
described below.
(1) Joy Stick & GUI Arrow Buttons:
[0361] A direction indicated by the joy stick and a GUI arrow
button is assumed to be a direction in which the operator wishes to
view, so that the stage is moved in the direction opposite to the
indicated direction.
Example )
[0362] Indicated Direction: Right . . . Stage Moving Direction:
Left (an image moves to the left=the field of view moves to the
right)
[0363] Indicated Direction: Upward . . . Stage Moving Direction:
Downward (an image moves downward=the field of view moves
upward)
(2) Direct Entry of Coordinates on GUI:
[0364] Coordinates directly entered on the GUI are regarded as a
location at which the operator wishes to view on the wafer
coordinate system, so that the stage is moved such that the
coordinate on a wafer are displayed at the center of a captured
image.
[0365] In the apparatus described in connection with FIG. 14, a
procedure is taken to mount a correction ring on the electrostatic
chuck, and position a wafer such that the wafer fits in the inner
diameter of the correction ring. Therefore, in the inspection
apparatus illustrated in FIG. 15, a procedure is taken to mount a
correction ring on a wafer in the load lock chamber 22.cndot.1,
integrally transfer the wafer mounted with the correction ring into
the sample chamber 21.cndot.7, and place the wafer mounted with the
correction ring on the electrostatic chuck on the stage. A feature
for implementing the foregoing procedure may be an elevator
mechanism for moving up and down an elevator to pass a wafer from
the atmosphere transfer robot to the vacuum transfer robot, as
shown in FIG. 25. The following description will be focused on a
procedure of transferring a wafer using this mechanism.
[0366] As illustrated in FIG. 25(A), the elevator mechanism
disposed in the load lock chamber has a plurality of stages of
correction ring support shelves (two stages in the figure) arranged
for movement in the vertical direction. An upper correction ring
support shelf 181 and a lower correction ring support shelf 182 are
fixed to a first base 184 which is moved up and down through
rotations of a first motor 183. Thus, the rotation of the first
motor 183 causes the first base 184 and the upper and lower
correction ring support shelves 181, 182 to move up or down.
[0367] Carried on each correction ring support shelf are correction
rings 185 each having an inner diameter suited to a particular size
of a wafer. There are two types of correction rings 185 provided
for 200-mm wafers and 300-mm wafers, respectively. These correction
rings have the same outer diameter. Such use of correction rings
having the same outer diameter results in compatibility, allowing
correction rings for 200-mm wafers and for 300-mm wafers to be
stored in a free combination in the load lock chamber. In other
words, for a line on which 200-mm wafers and 300-mm wafers flow in
mixture, the upper shelf is dedicated to correction rings for
300-mm wafers, while the lower shelf is dedicated to correction
rings for 200-mm wafers, such that a test can be conducted for
whichever wafer appears, thus supporting any wafer in a flexible
manner. On the other hand, for a line on which wafers of the same
size flow, the upper and lower shelves are dedicated to correction
rings for 200-mm or 300-mm wafers, so that wafers on the upper and
lower shelves can be alternately tested to improve the
throughput.
[0368] A second motor 186 is carried on the first base 184, while a
second base 187 is attached to the second motor 186 such that the
second base 187 can be moved up and down. An upper wafer support
shelf 188 and a lower wafer support shelf 189 are fixed on the
second base 187. With this structure, the rotation of the second
motor 186 causes the second base 187 and upper and lower wafer
support shelves 188, 189 to integrally move up or down.
[0369] Bearing the foregoing in mind, a wafer W placed on the hand
of the atmosphere transfer robot 161 is introduced into the load
lock chamber 171, as illustrated in FIG. 25(A). Next, as
illustrated in FIG. 25(B), the second motor 186 is rotated in a
first direction, causing the wafer support shelves 188, 189 to move
up. Then, the wafer W is placed on the upper wafer support shelf
188. In this way, the wafer W is moved from the atmospheric
transfer robot 161 to the wafer support shelf 188. Subsequently, as
illustrated in FIG. 25(C), the atmosphere transfer robot 161 is
retracted, and the second motor 186 is rotated in the direction
opposite to the first direction, as illustrated in FIG. 25(D), when
the atmosphere transfer robot 161 has been retracted, causing the
wafer support shelves 188, 189 to move down. In this way, the wafer
W is placed on the upper correction ring 185.
[0370] Next, as illustrated in FIG. 25(E), the hand of the vacuum
transfer robot 161 is introduced into the load lock chamber 171,
and stopped below the correction ring 185. In this state, the first
motor 183 is rotated to move down the first base 184, upper and
lower correction ring support shelves 181, 182, second motor 186,
and upper and lower wafer support shelves 188, 189, as illustrated
in FIG. 25(F). In this way, the correction ring 185 and wafer W
placed on the upper wafer support shelf 188 can be carried on the
hand of the vacuum transfer robot 161, and introduced into the
sample chamber 167.
[0371] The operation for bringing a wafer which has undergone a
test in the sample chamber 167 back into the load lock chamber 164
is performed in a procedure reverse to the foregoing. A wafer
carried on a wafer support shelf together with a correction ring by
the vacuum transfer robot is transferred to a correction ring
support shelf, next to the wafer support shelf, and finally on the
atmosphere transfer robot. While the foregoing description has been
made-of the wafer passing operation on the upper shelf with
reference to FIG. 25, a similar operation can be accomplished as
well on the lower shelf by adjusting the hands of the atmosphere
transfer robot 161 and vacuum transfer robot 166 in height. By
appropriately switching the heights for the hands of the atmosphere
transfer robot 161 and vacuum transfer robot 166 in the foregoing
manner, it is possible to alternate the introduction of an untested
wafer from one shelf into the sample chamber and the removal of a
tested wafer from the sample chamber to the other shelf.
Loader 67
[0372] The loader 67 (FIG. 6) comprises a robot-based first
transfer unit 78 disposed in the housing 80 of the mini-environment
device 63, and a robot-based second transfer unit 78' disposed in
the second loading chamber 108.
[0373] The first transfer unit 78 has a multi-node arm 191 for
rotation about an axis O.sub.1-O.sub.1 relative to a driver 190.
While an arbitrary structure may be applied to the multi-node arm,
this embodiment employs the multi-node arm 191 which has three
parts attached for rotation relative to each other. A part of the
arm 191 of the first transfer unit 78, i.e., a first part closest
to the driver 190 is attached to a shaft 192 which can be rotated
by a driving mechanism (not shown) in a general-purpose structure
arranged in the driver 190. The arm 191 is rotatable about the axis
O.sub.1-O.sub.1 by the shaft 192, and is telescopical in a radial
direction relative to the axis O.sub.1-O.sub.1 as a whole through
relative rotations among the parts. At the leading end of the third
part furthest away from the shaft 192 of the arm 191, a chuck 194
is attached for chucking a wafer, such as a mechanical chuck in a
general-purpose structure, an electrostatic chuck or the like. The
driver 190 is vertically movable by an elevating mechanism 195 in a
general-purpose structure.
[0374] In this first transfer unit 78, the arm 191 extends toward
one of two cassettes c held in the cassette holder 10 in a
direction M1 or M2 (FIG. 7), and a wafer W stored in the cassette c
is carried on the arm, or is chucked by the chuck (not shown)
attached at the leading end of the arm for removal. Subsequently,
the arm is retracted (to the state illustrated in FIG. 7), and the
arm is rotated to a position at which the arm can extend toward the
pre-aligner 96 in a direction M3, and is stopped at this position.
Then, the arm again extends to the pre-aligner 96 to transfer the
wafer held by the arm thereto. After receiving the wafer from the
pre-aligner 96 in a manner reverse to the foregoing, the arm is
further rotated and stopped at a position at which the arm can
extend toward the first loading chamber 107 (in a direction M4),
where the wafer is passed to a wafer receiver 196 within the first
loading chamber 107. It should be noted that when a wafer is
mechanically chucked, the wafer should be chucked in a peripheral
zone (in a range approximately 5 mm from the periphery). This is
because the wafer is formed with devices (circuit wires) over the
entire surface except for the peripheral zone, so that if the wafer
were chucked at a portion inside the peripheral zone, some devices
would be broken or defects would be produced.
[0375] The second transfer unit 78' is basically the same as the
first transfer unit 78 in structure, and differs only in that the
second transfer unit 78' transfers a wafer W between the wafer rack
124 and the carrying surface of the stage apparatus 66.
[0376] The first and second transfer units 78, 78' transfer wafers
from the cassette c held in the cassette holder onto the stage
apparatus 66 disposed in the working chamber 97 and vice versa
while holding the wafer substantially in a horizontal posture.
Then, the arms of the transfer units 78, 78' are moved up and down
only when a cassette is extracted from the cassette c and loaded
into the same, when a wafer is placed on the wafer lack and is
extracted from the same, and when a wafer is placed on the stage
apparatus 66 and removed from the same. Therefore, the transfer
units 78, 78' can smoothly move even a large wafer which may have a
diameter of, for example, 30 cm.
[0377] Now, a description will be made in order of the transfer of
a wafer from the cassette c supported by the cassette holder 62 to
the stage apparatus 66 disposed in the working chamber 97 in the
inspection system 1 having the configuration described above.
[0378] The cassette holder 62 for use in the inspection system 1
may have an appropriate structure either when cassettes are
manually set or when cassettes are automatically set, as mentioned
above. In this embodiment, as the cassette c is set on the up/down
table 71, the up/down table 71 is moved down by the elevating
mechanism 72 to bring the cassette c into alignment to the access
port 91. As the cassette c is in alignment to the access port 91, a
cover (not shown) disposed on the cassette c is opened, whereas a
cylindrical cover is arranged between the cassette c and the access
port 91 of the mini-environment device 63 to block the cassette c
and mini-environment space 63 from the outside. When the
mini-environment device 63 is equipped with a shutter device for
opening/closing the access port 91, the shutter device is operated
to open the access port 91.
[0379] On the other hand, the arm 191 of the first transfer unit 78
remains oriented in either the direction M1 or M2 (in the direction
M1 in this description), and extends to receive one of wafers
stored in the cassette c with its leading end as the access port 91
is opened.
[0380] Once the arm 191 has received a wafer, the arm 191 is
retracted, and the shutter device (if any) is operated to close the
access port 91. Then, the arm 191 is rotated about the axial line
O.sub.1-O.sub.1 so that it can extend in the direction M3. Next,
the arm 191 extends to transfer the wafer carried on the leading
end thereof or chucked by a chuck onto the pre-aligner 96 which
determines a direction in which the wafer is rotated (direction
about the center axis perpendicular to the surface of the wafer)
within a predetermined range. Upon completion of the positioning,
the first transfer unit 78 retracts the arm 191 after the wafer is
received from the pre-aligner 96 to the leading end of the arm 191,
and takes a posture in which the arm 191 can be extended in the
direction M4. Then, the door 197 of the shutter device 119 is moved
to open the access ports 226, 436, permitting the arm 191 to place
the wafer on the upper shelf or lower shelf of the wafer rack 124
within the first loading chamber 107. It should be noted that
before the shutter device 119 opens the access ports to pass the
wafer to the wafer rack 124, the opening 199 formed through the
partition 121 is hermetically closed by the door 122 of the shutter
device 123.
[0381] In the wafer transfer process by the first transfer unit 78,
clean air flows in a laminar state (as a down flow) from the gas
supply unit 231 disposed in the housing body 84 of the
mini-environment device 63, for preventing dust from sticking to
the upper surface of the wafer during the transfer. Part of air
around the transfer unit (in this embodiment, approximately 20% of
the air supplied from the gas supply unit 88, which is mainly
contaminated) is aspired from the suction duct 95 of the discharger
24 for emission out of the housing body 84. The remaining air is
recovered through the recovery duct 89 arranged on the bottom of
the housing body 84, and again returned to the gas supply unit
88.
[0382] As a wafer is placed on the wafer rack 196 within the first
loading chamber 107 of the loader housing 65 by the first transfer
unit 78, the shutter device 119 is closed to hermetically close the
loading chamber 107. Then, the loading chamber 107 is brought into
a vacuum atmosphere by expelling the air within the loading chamber
107, filling an inert gas in the loading chamber 107, and then
discharging the inert gas. The vacuum atmosphere in the loading
chamber 107 may have a low degree of vacuum. As the degree of
vacuum has reached a certain level in the loading chamber 107, the
shutter device 123 is operated to open the access port 121, which
has been hermetically closed by the door 122, and the arm 200 of
the second transfer unit 78' extends to receive one wafer from the
wafer receiver 196 with the chuck at the leading end thereof
(placed on the leading end or chucked by a chuck attached to the
leading end). As the wafer has been received, the arm 200 is
retracted, and the shutter device 123 is again operated to close
the access port 199 with the door 122. It should be noted that
before the shutter device 123 opens the access port 199, the arm
200 has previously taken a posture in which it can extend toward
the wafer rack 196 in a direction N1. Also, as described above,
before the shutter device 123 opens the access port 199, the
shutter device 120 closes the access ports 116, 106 with the door
201 to block communications between the second loading chamber 108
and the working chamber 97, and the second loading chamber 108 is
evacuated.
[0383] As the shutter device 123 closes the access port 199, the
second loading chamber 108 is again evacuated to a degree of vacuum
higher than that of the first loading chamber 107. In the meantime,
the arm 191 of the second transfer unit 78 is rotated to a position
from which the arm 191 can extend toward the stage apparatus 66
within the working chamber 97. On the other hand, in the stage
apparatus 66 within the working chamber 97, the Y-table 202 is
moved upward, as viewed in FIG. 13, to a position at which the
center line X.sub.0-X.sub.0 of the X-table 203 substantially
matches an X-axis line X.sub.1-X.sub.1 which passes the axis of
rotation O.sub.2-O.sub.2 of the second transfer unit 78'. Also, the
X-table 203 has moved to a position close to the leftmost position,
as viewed in FIG. 2, and is waiting at this position. When the
degree of vacuum in the second loading chamber 108 is increased to
a level substantially identical to that of the working chamber 97,
the door 201 of the shutter device 120 is moved to open the access
ports 116, 106, and the arm extends so that the leading end of the
arm, which holds a wafer, approaches the stage apparatus 66 within
the working chamber 97. Then, the wafer W is placed on the carrying
surface 130 of the stage apparatus 66. Once the wafer W has been
placed on the stage apparatus 66, the arm is retracted, and the
shutter device 120 closes the access ports 116, 106.
[0384] The foregoing description has been made of a sequence of
operations until a wafer W in the cassette c is transferred to the
working chamber 97 and placed on the carrying surface 130 of the
stage apparatus 66. For returning a wafer W which has undergone a
test from the stage apparatus 66 to the cassette c, operations
reverse to the foregoing are performed. Also, since a plurality of
wafers are placed on the wafer rack 196, the first transfer unit
can transfer a wafer between the cassette c and the wafer rack 196
while the second transfer unit 78' is transferring a wafer between
the wafer rack 196 and the stage apparatus 66. Consequently,
operations associated with the test can be efficiently
conducted.
Pre-Charge Unit 69
[0385] The pre-charge unit 69 is disposed in close proximity to the
barrel 204 of the electro-optical system 68 within the working
chamber 97, as previously shown in FIG. 6. Since the inspection
system 1 of the present invention irradiates a wafer with electron
beams for scanning to test a device pattern and the like formed on
the surface of the wafer, the wafer can be charged on the surface
(charge-up) depending on conditions such as the material of the
wafer, energy of irradiated electron beams, and the like. Further,
the wafer surface may include a region which is more charged and a
region which is less charged. In addition, while information on
secondary electrons or the like generated by irradiation of
electron beams is used for analyzing the wafer surface, possible
variations in the amount of charge on the wafer surface may cause
the information on the secondary electrons to include variations as
well, thereby failing to provide accurate images.
[0386] To prevent such variations in charge, the pre-charge unit 69
is provided in this embodiment. The pre-charge unit 69 includes a
charged particle irradiating unit 205 which irradiates charged
particles to a wafer before primary electron beams are emitted for
testing, thereby eliminating variations in charge. How the wafer
surface is charged can be detected by previously forming an image
of the wafer surface using the electro-optical system 68, and
evaluating the image. Then, the irradiation of charged particles
from the charged particle irradiating unit 205 is controlled based
on the detected charging state. The pre-charge unit 69 may
irradiate blurred primary electron beams.
Alignment Control Unit 70
[0387] The alignment control unit 70 aligns a wafer W to the
electro-optical system 68 using the stage apparatus 66. The
alignment control unit 70 is configured to control a low
magnification alignment (alignment with a lower magnification than
the electro-optical system 68) which is a rough alignment of a
wafer through a wide field observation using the optical microscope
206 (FIGS. 6 and 26); a high magnification alignment for a wafer
using the electro-optical system 68; focus adjustment; setting of
an area under inspection; pattern alignment; and the like. It
should be noted that a wafer is tested at a low magnification as
mentioned above because for automatically inspecting patterns on a
wafer, an alignment mark must be readily detected by electron beams
when the wafer is aligned by observing the patterns on the wafer in
a narrow field of view using electron beams.
[0388] The optical microscope 206 is installed within the main
housing 64, but may be movably disposed within the main housing 64.
A light source (not shown) for operating the optical microscope 206
is also disposed within the main housing 64. Further, the
electro-optical system involved in observations at high
magnification shares components (primary optical system and
secondary optical system) of the electro-optical system 68.
[0389] FIG. 26 generally illustrates the configuration of the
alignment control unit 70. For observing a site under observation
on a wafer W at a low magnification, the site under observation on
the wafer W is moved into the field of view of the optical
microscope 206 by moving the X-stage or Y-stage of the stage
apparatus 66. The wafer W is viewed in a wide field of view using
the optical microscope 206, and the site under observation on the
wafer W is displayed on a monitor 208 through a CCD 207 to roughly
determine where the site under observation is found. In this event,
the magnification of the optical microscope 206 may be gradually
changed from a low magnification to a high magnification.
[0390] Next, the stage apparatus 66 is moved by a distance
corresponding to a spacing .delta.x between the optical axis of the
electro-optical system 68 and the optical axis of the optical
microscope 206, thereby moving the site under observation on the
wafer W, which has been previously determined using the optical
microscope 206, into the field of view of the electro-optical
system 68. In this event, since the distance .delta.x between the
axial line O.sub.3-O.sub.3 of the electro-optical system 68 and the
optical axis O.sub.4-O.sub.4 of the optical microscope 206 has been
previously known (while both are shifted only in the X-direction in
this embodiment, they may be shifted in the Y-direction), the site
under observation can be moved to a viewing position of the
electro-optical system 68 if the wafer W is moved by the distance
.delta.x. After the site under observation has been moved to the
viewing position of the electro-optical system 68, the site under
observation is imaged at a high magnification by the
electro-optical system, and the resulting image is stored or
displayed on a monitor 209.
[0391] After the site under observation of the wafer is displayed
at a high magnification by the electro-optical system 68 as
described above, a displacement of the wafer in the rotating
direction relative to the center of rotation of the rotatable table
128 of the stage apparatus 66, i.e., a shift .delta..theta. of the
wafer in the rotating direction relative to the optical axis
O.sub.3-O.sub.3 of the electro-optical system is detected by a
known method, and a displacement of a predetermined pattern is
detected in the X-axis and Y-axis directions relative to the
electro-optical system 68. Then, the operation of the stage
apparatus 50 is controlled to align the wafer based on the detected
values, data on a test mark separately attached on the wafer, or
data related to the shapes of the patterns on the wafer. In the
following, an alignment procedure will be described in greater
detail.
[0392] Dies on a wafer loaded on the stage are arranged in a
direction which is not necessarily coincident with a scanning
direction of a TDI camera (see FIG. 27). To make them coincident,
the wafer must be rotated on the .theta. stage through an operation
called "alignment" (FIG. 28). An alignment recipe saves alignment
execution conditions after a wafer is loaded on the stage.
[0393] In addition, a die map (FIG. 29) is also created for
indicating the arrangement of dies upon execution of the alignment,
and a die map recipe including the size of dies, the position of an
origin die (which serves as the origin for indicating the location
of a particular die), and the like is stored.
[0394] The alignment (positioning) procedure involves first making
a rough alignment at a low magnification with an optical
microscope, making a detailed alignment at a high magnification
with the optical microscope, and finally making a fine alignment
using an EB image.
A. Imaging at Low Magnification Using Optical Microscope:
(1) <Specify First, Second, Third Searched Dies and
Template>
[0395] (1-1) Specify First Searched Die and Template:
[0396] The user moves the stage such that the lower left corner of
a die located in a lower region of a wafer is positioned near the
center of the camera, and captures a template image for pattern
matching after determining the position. This die is referenced for
the positioning, and the coordinate at the lower left corner are
the coordinate of a characteristic point. From then on, this
template image is used for pattern matching to measure the
coordinate of the precise location of an arbitrary die on the
wafer. An image selected for the template image must be a unique
pattern within a search region.
[0397] While the lower left corner is defined to be the position at
which the template image for pattern matching is captured in this
embodiment, the characteristic point is not limited to the lower
left corner, but may be an arbitrary location within a die.
Generally, however, it is easier to use a corner is to identify the
coordinate than a point located within a die or on a side of the
die, so that one of the four corners is preferably selected.
Likewise, in this embodiment, a template image for pattern matching
is captured for a die located in a lower region of a wafer, but it
goes without saying that an arbitrary die may be selected to
facilitate the alignment.
[0398] (1-2) Specify Second Searched Die:
[0399] A die next to the first searched die on the right side is
chosen to be a second searched die, and the user moves the stage
such that the lower left corner of the second searched die is
positioned near the center of the camera. After determining the
position, the pattern matching is automatically performed using the
template image captured in the aforementioned section (1-1) to
acquire precise coordinate values for a pattern of the second
searched die which is coincident with the template image specified
in the first searched die.
[0400] While the die adjacent to the first searched die on the
right side is chosen to be the second searched die for purposes of
description in this embodiment, the second searched die of the
present invention is not limited to this die, as a matter of
course. In essence, the selection may be made for a point at which
a positional relationship of dies in the row direction can be more
precisely found from the reference point at which precise
coordinate has been found for the position of the characteristic
point. Therefore, a die adjacent to the first searched die on the
left side may be chosen to be the second searched die.
(1-3) Specify Third Searched Die:
[0401] A die immediately above the second searched die is chosen to
be a third searched die, and the user moves the stage such that the
lower left corner of the third searched die is positioned near the
center of the camera. After determining the position, the pattern
matching is automatically performed using the template image
captured in the aforementioned section (1-1) to acquire precise
coordinates for a pattern of the third searched die which are
coincident with the template image specified in the first searched
die.
[0402] While the die immediately above the first searched die on
the right side is chosen to be the third searched die for purposes
of description in this embodiment, the third searched die of the
present invention is not limited to this die, as a matter of
course. In essence, the selection may be made such that a
positional relationship including a distance to the coordinate of a
particular point of a die in the column direction can be found,
with reference to the die at which precise coordinate has been
found for the position of the characteristic point. Therefore, a
die immediately above the second searched die may be chosen to be
the third searched die.
(2) <Y-Direction Low Magnification Pattern Matching>
[0403] (2-1) Moving amounts (dX, dY) to the immediately above die
are calculated from the relationship between the pattern match
coordinate (X2, Y2) of the second searched die and the pattern
match coordinate (X3, Y3) of the third searched die:
dX=X3-X2
dY=Y3-Y2
[0404] (2-2) The stage is moved to coordinate (XN, YN) at which a
pattern of a die immediately above the first searched die will (be
expected to) exist using the calculated moving amount (dX, dY).
XN=X1+dX
YN=Y1+dY
where (X1, Y1) are the coordinate values of a pattern of the first
searched die.
[0405] (2-3) Precise coordinate values (XN, YN) of a pattern
currently under observation are captured by imaging at a low
magnification with the optical microscope after the stage has been
moved and executing the pattern matching using the template image,
and one is set to the initial value for the number of detected dies
(DN).
[0406] (2-4) Moving amount value (dX, dY) are calculated from the
coordinate (X1, Y1) of the pattern of the first searched die to the
coordinate (XN, YN) of the pattern which is currently being
imaged.
dX=XN-X1
dY=YN-Y1
[0407] (2-5) The stage is moved from the first searched die by
moving amounts (2*dX, 2*dY) twice as much as the calculated moving
amounts (dX, dY).
[0408] (2-6) The precise coordinate (XN, YN) of the pattern
currently under observation are updated by imaging at a low
magnification with the optical microscope after the stage has been
moved, and executing the pattern matching using the template image,
and the number of detected dies is increased by a factor of two.
See FIG. 30 for this operation.
[0409] (2-7) Steps (2-4) to (2-6) are executed in repetition toward
the upward direction on the wafer until a previously specified
Y-coordinate value is exceeded.
[0410] While this embodiment has been described in connection with
an exemplary scenario in which a double moving amount is repeated
in order to increase the accuracy, reduce the number of times of
processing (number of repetitions), and reduce the processing time,
a high integer magnification of more than two, such as three times
or four times may be used for execution on the condition that no
problem occurs in accuracy and the processing time is preferably
further reduced. Conversely, the movement may be repeated with a
fixed moving amount for further increasing the accuracy, on the
condition that no problem occurs. In either case, it goes without
saying that this should be reflected to the number of detected
dies.
(3) <.theta. Rotation at Low Magnification of Optical
Microscope>
[0411] (3-1) A rotating amount (.theta.) and a Y-direction die size
(YD) are calculated using the moving amount from the pattern
coordinate (X1, Y1) of the first searched die to the precise
coordinate (XN, YN) of the pattern of the finally searched die, and
the number (DN) of dies so far detected (see FIG. 31).
dX=XN-X1
dY=YN-Y1
.theta.=tan.sup.-1(dX/dY)
YD=((dX).sup.2+(dY).sup.2).sup.1/2/DN
[0412] (3-2) The .theta. Stage is Rotated by the Calculated
Rotating Amount (.theta.).
B. Imaging at High Magnification Using Optical Microscope:
[0413] (1) A procedure similar to (1) of the imaging at a low
magnification is executed at a high magnification using the optical
microscope.
[0414] (2) A procedure similar to (2) of the imaging at a low
magnification is executed at a high magnification using the optical
microscope.
[0415] (3) A procedure similar to (3) of the imaging at a low
magnification is executed using the optical microscope.
[0416] (4) <Check Tolerance After Optical Microscope High
Magnification .theta. Rotation>
[0417] (4-1) [Specify First Searched Die and Template for Imaging
at High Magnification with Optical Microscope]
[0418] The coordinate (X'1, Y'1) of the first searched die after
the rotation is calculated from the coordinate (X1, Y1) before the
rotation and the rotating amount (.theta.). The stage is moved to
the coordinate (X'1, Y'1). After determining the position, a
template image is captured for pattern matching.
X'1=x.sub.1*cos .theta.-y.sub.1*sin .theta.
Y'1=x.sub.1*sin .theta.+y.sub.1*cos .theta.
[0419] (4-2) Pattern Matching in Y-Direction at High Magnification
with Optical Microscope
[0420] The stage is moved in the-Y-direction by dY from the
coordinate (X'1, Y'1) of the first searched die after the rotation,
and the pattern matching is executed to acquire precise coordinate
(XN, YN) of the pattern currently under observation.
[0421] (4-3) From Coordinates (X'1, Y'1) of First Searched Die
after Rotation to Coordinate of Pattern Currently under Imaging
[0422] Moving amounts (dX, dY) to the coordinate (XN, YN) is
calculated.
dX=XN-X'1
dY=YN-Y'1
[0423] (4-4) The stage is moved from the first searched die by
moving amounts (2*dX, 2*dY) twice as much as the calculated moving
amounts (dX, dY).
[0424] (4-5) The precise coordinate (XN, YN) of the pattern
currently under observation is updated by imaging at a low
magnification with the optical microscope after the stage has been
moved, and executing the pattern matching using the template
image.
[0425] (4-6) Steps (4-3) to (4-5) are executed in repetition in the
upward direction on the wafer until a previously specified
Y-coordinate value is exceeded.
[0426] (4-7) Calculate Rotating Amount of .theta.:
[0427] The rotating amount (.theta.) is calculated using a moving
amount from the coordinate (X'1, Y'1) of the first searched die
after the rotation to the precise coordinate (XN, YN) of a pattern
of a finally searched die.
dX=XN-X1
dY=YN-Y1
.theta.=tan..sup.-1(dX/dY)
[0428] (4-8) Optical Microscope High Magnification .theta.
Tolerance Check:
[0429] Confirmation is made as to whether the rotating amount
(.theta.) calculated in (4-7) falls within a predefined value. If
it does not, steps (4-1) to (4-8) are executed again after the
.theta. stage is rotated using the calculated rotating amount
(.theta.). However, when the rotating amount (.theta.) does not
fall within the tolerance even after repeating steps (4-1) to (4-8)
a predefined number of times, the processing is aborted, and it is
determined that an error has occurred.
C. Alignment Using EB Image
(1) <Specify Y Search First Die and EB Template>
[0430] A procedure similar to (1) of the imaging at a high
magnification with the optical microscope is executed using an EB
image.
(2) <EB Y-Direction Pattern Matching>
[0431] A procedure similar to (2) of the imaging at a high
magnification with the optical microscope is executed using an EB
image.
(3) <EB .theta. Rotation>
[0432] A procedure similar to (3) of the imaging at a high
magnification with the optical microscope is executed using an EB
image.
(4) <EB Tolerance Check After Rotation of .theta.>
[0433] A procedure similar to (4) of the imaging at a high
magnification with the optical microscope is executed using an EB
image.
[0434] (5) Steps (1) to (4) are executed using an EB image at a
high magnification as required.
[0435] (6) An approximate value for the die size (XD) in the
X-direction is calculated from the coordinate (X1, Y1) of the first
searched die and the coordinate (X2, Y2) of the second searched
die.
dX=X2-X1
dY=Y2-Y1
XD=((dx).sup.2+(dy).sup.2).sup.1/2
D. Creation of Die Map Recipe
(1) <Specify X-Search First Die and EB Template>
[0436] The user moves the stage such that the lower left corner of
the die located at the left end of the wafer is positioned near the
center of a TDI camera, and acquires a template image for pattern
matching after determining the position. Selected for this template
image should be an image which is a unique pattern within a search
region.
(2) <EB X-Direction Pattern Matching>
[0437] (2-1) The stage is moved to the coordinate (X1+XD, Y1) at
which a pattern of a die on the right side of the first searched
die in the X-direction will (be expected to) exist, using an
approximate value (XD) of the die size in the X-direction.
[0438] (2-2) After the stage is moved, an EB image is captured by
the TDI camera. Precise coordinate (XN, YN) of a pattern currently
under observation are acquired by executing the pattern matching
using the template image, and one is set to an initial value for
the number of detected dies (DN).
[0439] (2-3) Moving amounts (dX, dY) are calculated from the
coordinate (X1, Y1) of the pattern on the X-search first die to the
coordinate (XN, YN) of the pattern which is currently being
imaged.
dX=XN-X1
dY=YN-Y1
[0440] (2-4) The stage is moved from the first searched die in the
X-direction by moving amounts (2*dX, 2*dY) which is twice as much
as the calculated moving amounts (dX, dY).
[0441] (2-5) The precise coordinate (XN, YN) of the pattern
currently under observation is updated by capturing an EB image
with the TDI camera after the stage has been moved, and executing
the pattern matching using the template image, and the number of
detected dies is increased by a factor of two.
[0442] (2-6) Steps (2-3) to (2-5) are repeatedly executed in the
right direction on the wafer until a previously specified
X-coordinate value is exceeded.
(3) <Calculation of X-Direction Slope>
[0443] A stage straight-going error (.PHI.) and X-direction die
size (XD) are calculated using the moving amount from the
coordinate (X1, Y1) of the pattern on the first searched die in the
X-direction to the precise coordinate value (XN, YN) of the pattern
on the finally searched die, and the number (DN) of dies so far
detected.
dX=XN-X1
dY=YN-Y1
.PHI.=tan.sup.-1(dX/dY)
XD=((dX).sup.2+(dy).sup.2).sup.1/2/DN
(4) Creation of Die Map
[0444] The X-direction die size (XD) thus calculated is combined
with a Y-direction die size (YD) found during the calculation of
the rotating amount (.theta.) to create a die map (ideal die
arrangement information). The die map permits an ideal arrangement
for dies to be found. On the other hand, any die on the substrate
is affected, for example, by mechanical errors of the stage (errors
in parts such as guides, and errors in assembly), errors of the
interferometer (for example, due to the assembly of mirrors and the
like), distorted images due to charge-up, so that all dies cannot
be observed for an ideal arrangement, but the test should be
conducted while finding errors between the actual locations of dies
and an ideal arrangement on the die map, and automatically
correcting the errors in consideration thereof.
E. Focus Recipe Creation Procedure
[0445] Next, description will be made of a procedure of creating a
focus recipe. The focus recipe stores information on an optimal
focus position at a position of a mark on a flat surface of a
sample such as a substrate, and information on a variety of
conditions related to the focus position in a predetermined format
such as a table. On a focus map recipe, the focus condition is set
only for specified locations on a wafer, and focus values between
the specified locations are linearly interpolated (see FIG. 32).
The focus recipe creation procedure is as follows.
[0446] (1) A die subjected to a focus measurement is selected from
the die map.
[0447] (2) A focus measurement point is set within a die.
[0448] (3) The stage is moved to each point at which the focus
value (CL12 voltage) is manually adjusted based on the image and
contrast value.
[0449] The die map created through the alignment processing shows
ideal positional information calculated from the coordinate of the
dies at both ends of a wafer, and errors can occur due to a variety
of factors between the locations of dies on the die map and actual
locations of dies (see FIG. 33). A procedure for creating a
parameter for absorbing the errors is called "fine alignment," and
a fine alignment recipe error information of the die map (ideal die
arrangement information) and actual locations of dies. The
information set herein is used during a defect detection. In the
fine alignment recipe, errors are measured only for those dies
which are specified on the die map, and errors between the
specified dies are linearly interpolated.
F. Fine Alignment Procedure
[0450] (1) Dies subjected to error measurement for fine alignment
are specified from the die map.
[0451] (2) A reference die is selected from the die subjected to
error measurement, and the location of this die is defined to be a
point at which there is no error with respect to the die map.
[0452] (3) The lower left corner of the reference die is imaged by
the TDI camera to capture a template image for pattern matching
(however, a unique pattern within a search region is selected as
the template image).
[0453] (4) Lower left (on the die map) coordinate (X0, Y0) of a
nearby die subjected to error measurement are acquired, and the
stage is moved thereto. After the movement, the die is imaged by
the TDI camera, and precise coordinate (X, Y) is acquired by
executing the pattern matching using the template image captured in
step (3).
[0454] (5) Errors between the coordinate (X, Y) acquired through
the pattern matching and coordinate (X0, Y0) on the die map are
saved.
[0455] (6) Steps (4)-(5) are executed for all the dies subjected to
error measurement.
[0456] A further detailed description will be made on a defect
detecting apparatus for processing data generated by the
electro-optical system 70 to acquire image data and for detecting
defects on a semiconductor wafer based on the acquired image data
in accordance with the present invention. Generally, the inspection
apparatus using electron beams, i.e., the electro-optical system 70
is expensive and presents a lower throughput than other process
apparatuses. For this reason, the inspection apparatus is currently
utilized after important processes which are thought to have the
most need of the test (for example, etching, deposition, CMP
(chemical mechanical polishing) planarization, and the like) or in
part of a wiring process which involves finer wires, i.e., one or
two steps of the wiring process, in a gate wiring step in the
pre-process, and the like. In particular, it is important to find
defective shapes and electric defects of wires having a design rule
of 100 nm or less, via holes having diameters of 100 nm or less,
and the like, and to feed the found defects back to associated
processes.
[0457] As described above, a wafer to be tested is transferred by
the atmosphere transfer system and vacuum transfer system, aligned
on the highly precise stage apparatus (X-Y stage) 66, and then
fixed by an electrostatic chucking mechanism or the like. Then, in
a defect inspection process, an optical microscope is used to
confirm the location of each die and detect the height of each
location, as required, and such data is stored. The optical
microscope is also used to capture an optical microscopic image of
desired sites such as defects and to compare electron beam images.
Next, conditions are set for the electro-optical system, and an
electron beam image is used to modify the information set by the
optical microscope to improve accuracy.
[0458] Next, information on recipes is entered to the apparatus
depending on the type of wafer (after which process, whether the
wafer size is 200 mm or 300 mm, and the like). Subsequently, after
specifying a inspection place, setting the electro-optical system,
setting inspection conditions, and the like, a defect test is
normally conducted in real time while images are captured. A
comparison of cells to one another, a comparison between dies, and
the like are performed by a high speed information processing
system which has associated algorithms installed therein, and the
results are output to a CRT or the like, and stored in a memory, as
required.
[0459] FIG. 34 illustrates a basic flow of the defect test. First,
after transfer of wafers including an alignment operation 211, the
recipes are created for setting conditions related to the test, and
the like (212). While at least one type of recipe is needed for
each wafer under inspection, a plurality of recipes may be created
for a single wafer under inspection in order to support a plurality
of inspection conditions. Also, when there is a plurality of wafers
having the same pattern, the plurality of wafers may be tested in
accordance with a single recipe. A path 213 in FIG. 34 indicates
that when a test is conducted using recipes created in the past,
the creation of recipes is not required immediately before the
inspection operation.
[0460] In FIG. 34, the inspection operation 214 involves a test on
a wafer in accordance with the conditions described in the recipe
and a sequence. A defect is extracted immediately each time it is
found during the inspection operation through the following
operations which are executed substantially in parallel. [0461]
Defects are classified (215) to add extracted defect information
and defect classification information to a result output file.
[0462] An extracted defect image is added to a result output file
dedicated to images or to a file. [0463] Defect information such as
locations of extracted defects is displayed on an operation screen.
[0464] Upon completion of the test on a wafer-by-wafer basis, the
following operations are next executed substantially in parallel.
[0465] The result output file is closed and saved. [0466] When the
result of the test is requested through a communication from the
outside, the result of the test is sent. [0467] The wafer is
removed.
[0468] When the inspection system is set to continuously test
wafers, the next wafer under inspection is transferred, followed by
a repetition of the sequence of operations described above.
[0469] In the creation of recipes in FIG. 34, recipes created
therein include a file for setting conditions associated with the
test, and the like. The recipes can be saved as well, so that the
recipes may be used to set conditions at the time of or before a
test. The conditions associated with the test described in the
recipes include, for example, the following items: dies under
inspection; [0470] region to be tested within a die; [0471]
inspection algorithm; [0472] detecting conditions (required for
extracting defects, such as a test sensitivity); and [0473]
observation conditions (magnification, lens voltages, stage speed,
inspection order, and the like, which are required for
observation).
[0474] Among the test conditions listed above, the setting of dies
under inspection involves an operator specifying dies to be tested
on a die map screen displayed on the operation screen, as
illustrated in FIG. 35. In the example of FIG. 35, dies 1 near the
periphery of the wafer and dies 2 clearly determined as defective
in the pre-process are grayed out and removed from dies under
inspection, and the remaining dies are subjected to the test. The
alignment control unit 2 also has a function of automatically
specifying dies under inspection based on the distance from the
periphery of the wafer and information on good/fail of dies
detected in the pre-process.
[0475] An area under inspection within a die is specified by the
operator on a die internal test region setting screen displayed on
the operation screen, as illustrated in FIG. 36, using an input
device such as a mouse based on an image captured by an optical
microscope or an EB microscope. In the example of FIG. 36, an area
221 indicated by a solid line, and an area 222 indicated by a
broken line are set to be areas under inspection.
[0476] The area 221 includes substantially the entire die which is
set to be under inspection. In this event, an adjacent die
comparison method is employed for a test algorithm, and detailed
detection conditions and observation conditions for this area are
separately set. For the area 222, an array test is employed for a
test algorithm, and detection conditions and detailed observation
conditions for this area are separately set. Thus, a plurality of
areas under inspection can be set, and an appropriate test
algorithm and test sensitivity can be set for each of the areas.
Also, some areas under inspection can be overlapped, so that
different test algorithms can be simultaneously executed for the
same area.
[0477] In the inspection operation 214 in FIG. 34, a wafer under
inspection is sectioned in scanning widths, as illustrated in FIG.
37. The scanning width is substantially determined by the length of
a line sensor, but is set such that adjacent line sensors slightly
overlap in their respective edge portions. This is intended to
ensure a margin for determining the continuity between lines when
detected defects are totally processed at a final stage, and for an
alignment of images involved in a comparison test. An overlapping
amount is approximately 16 dots for a 2,048-dot line sensor.
[0478] FIG. 38 schematically illustrates scanning directions and
sequences. Specifically, a bi-directional operation (Operation A)
for reducing a test time, and a uni-directional operation
(Operation B) due to mechanical restrictions can be selected by the
operator. The control unit also has a function of automatically
processing and detecting to execute an operation which reduces the
amount of scanning for the test based on target die information
stored in the recipe. FIG. 39(A) shows an example of scanning which
is done when there is only one die under inspection, in which case
unnecessary scanning is omitted.
[0479] A test algorithm set by the recipe can be classified into a
cell test (array test) and a die test (random test).
[0480] As illustrated in FIG. 39(B), a die is divided into a cell
area 231 which has a periodic structure mainly used for memories,
and a random area 232 which does not have the periodic structure.
Since the cell area 231 having the periodic structure includes a
plurality of cells to be compared within the same die, the cells
within the same die can be tested using the cell test by comparing
them with one another. On the other hand, since the random area 232
cannot be compared within the same die, dies must be compared using
the die test.
[0481] The die test method is further classified as follows
depending on what is compared: [0482] an adjacent die comparison
method (Die-to-Die test); [0483] a reference die comparison method
(Die-to-Any Die test); and [0484] a CAD data comparison method (Cad
Data-to-Any Die test).
[0485] A scheme generally called a "golden template scheme" falls
under the basic die comparison method and CAD data comparison
method. In the reference die comparison method, a reference die is
used as a golden template, while in the CAD data comparison method,
CAD data is used as a golden template. The following description
will be made on the operation of the respective test
algorithms.
Cell Test (Array Test)
[0486] The cell test is applied to a test of a periodic structure.
A DRAM cell is an example which is suitable for the cell test.
[0487] The test involves comparing a reference image with an image
under inspection, and extracting differences therebetween as
defects. The reference image and image under inspection may be
digitized images or multi-valued images for improving the detection
accuracy.
[0488] While defects may be differences themselves between the
reference image and the image under inspection, a secondary
determination may be made in order to prevent erroneous detections
based on difference information such as the amount of detected
difference, a total area of pixels which present differences, and
the like.
[0489] In the cell test, the comparison of the reference image with
the image under inspection is made in units of structural periods.
Specifically, they may be compared in units of structural periods
while reading the images collectively captured by a CCD or the
like, or when the reference image comprises n units of structural
periods, the n units of structural period can be compared at the
same time.
[0490] FIG. 40 illustrates an exemplary method of generating a
reference image. FIG. 40 illustrates the generation of one
structural period unit because the following description will be
made on an exemplary comparison which is made on a unit-by-unit
basis. The number of periods can be increased to n in the same
method.
[0491] Assume that a test is conducted in a direction indicated by
an arrow A in FIG. 40. Assume also that period 4 is chosen to be a
period under inspection. Since the length of the period is entered
by the operator while viewing the image, periods 1-6 can be readily
recognized in FIG. 40.
[0492] The reference period image is generated by adding periods
1-3 immediately before the period under inspection and averaging
them in each pixel. Even if a defect is found in any of periods
1-3, the influence is not significant because these periods are
averaged. The reference period image thus generated is compared
with the period image 4 under inspection to extract defects.
[0493] When a period image 5 under inspection is next tested,
periods 2-4 are averaged to generate a reference period image.
Subsequently, a period image under inspection is generated from
images captured before the capturing of the period image under
inspection in a similar manner to continue the test.
Die Test (Random Test)
[0494] The die test is applied without limited by the structure of
die. The test involves comparing a reference image with an image
under inspection, and extracting differences therebetween as
defects. The reference image and image under inspection may be
digitized images or multi-valued images for improving the detection
accuracy. While defects may be differences themselves between the
reference image and the image under inspection, a secondary
determination may be made in order to prevent erroneous detections
based on difference information such as the amount of detected
difference, a total area of pixels which present differences, and
the like. The die test can be classified according to how a
reference image is generated. The following description will be
made on the operation of an adjacent die comparison method, a
reference die comparison inspection method, and a CAD data
comparison method which are included in the die test.
A. Adjacent Die Comparison Method (Die-Die Test)
[0495] The reference image represents a die adjacent to an image
under inspection. Two dies adjacent to the image under inspection
are compared to determine a defect. Specifically, referring to
FIGS. 41 and 42, the following steps are executed in a situation
where switches 245, 246 are set to connect a memory 241 and a
memory 242 of an image processing apparatus are connected to a path
244 of a camera 243.
[0496] a) A step of storing a die image 1 from the path 244 to the
memory 241 in accordance with a scanning direction S.
[0497] b) A step of storing a die image 2 from the path 244 to the
memory 242.
[0498] c) A step of capturing the die image 2 from a path 247
simultaneously with the foregoing step b), while comparing the
capture die image 2 with image data stored in the memory 241 which
is at the same relative position in the die to find
differences.
[0499] d) A step of saving the differences found in step c).
[0500] e) A step of storing a die image 3 from the path 244 to the
memory 241.
[0501] f) A step of capturing the die image from the path 247
simultaneously with the foregoing step e), while comparing the
captured die image 3 with image data stored in the memory 242 which
is at the same relative position in the die to find
differences.
[0502] g) A step of saving the differences found in step f).
[0503] h) A step of determining defects in the die image 2 from the
result saved in steps d) and g).
[0504] i) A step of subsequently repeating steps a) to h) in
consecutive dies.
[0505] Settings may be made to correct the two images to be
compared such that a position alignment, i.e., a difference in
position is eliminated in the two image before the differences are
found in steps c) and f). Alternatively, a correction may be made
to eliminate density alignment, i.e., a difference density. In some
cases, both processes may be required.
B. Reference Die Comparison Method (Die-Any Die Test)
[0506] The operator specifies a reference die. The reference die is
a die existing on a wafer, or a die image saved before the test.
First, the reference die is scanned or transferred to wave its
image in a memory for use as a reference image. Specifically, the
following steps are executed in FIGS. 42 and 43.
[0507] a) A step of selecting the reference die by the operator
from a die on a wafer under inspection or die images stored before
the test.
[0508] b) A step of setting the switch 245 and switch 246 such that
at least one of the memory 241 and memory 242 of an image
processing apparatus is connected to a path 244 from a camera 243
when a reference die exists on the wafer under inspection.
[0509] c) A step of setting the switch 245 and switch 246 such that
at least one of the memory 241 and memory 242 of the image
processing apparatus is connected to a path 249 from a memory 248
which stores a reference image that is the die image when the
reference die is a die image saved before the test.
[0510] d) A step of scanning the reference die, when it exists on
the wafer under inspection, and transferring a reference image,
which is a reference die image, to a memory of the image processing
apparatus.
[0511] e) A step of transferring a reference image, which is a
reference die image, to a memory of the image processing apparatus,
without the need for scanning, when the reference die is a die
image saved before the test.
[0512] f) A step of comparing an image generated by sequentially
scanning the image under inspection, the image in the memory to
which the reference image, i.e., the reference die image has been
transferred, and image data which is at the same relative position
in the die to find differences.
[0513] g) A step of determining defects from the differences found
in the foregoing step f).
[0514] h) A step of subsequently testing or inspecting the same
portions for a scanning position of the reference die and the die
origin of the die under inspection over the entire wafer, as
illustrated in FIG. 50, and repeating the foregoing steps d) to g)
while changing the scanning position of the reference die until the
entire die is tested.
[0515] Settings may be made to correct two images to be compared
such that a position alignment, i.e., a difference in position is
eliminated in the two images before the differences are found in
step f). Alternatively, a correction may be made to eliminate
density alignment, i.e., a difference density. In some cases, both
processes may be required. The reference die image stored in the
memory of the image processing apparatus in step d) or e) may be
the entire reference die, or a portion of the reference die which
is updated.
C. CAD Data Comparison Method (CAD Data-Any Die Test)
[0516] A certain image is created for use as a reference image from
CAD data which is the output of a CAD-based semiconductor pattern
designing process. The reference image may represent an entire die,
or part thereof which includes a portion under inspection. Also,
this CAD data is typically vector data which cannot be used as the
reference image unless the CAD data is converted to raster data
equivalent to image data captured by a scanning operation. Thus,
the following conversion process is executed in regard to the CAD
data processing operation.
[0517] a) Vector data, which comprises the CAD data, is converted
to raster data.
[0518] b) The foregoing step a) is performed in units of image
scanning width which is known by scanning the die under inspection
during a test.
[0519] c) The foregoing step b) converts image data which is at the
same relative position in the die as an image which is expected to
be captured by scanning the die under inspection.
[0520] d) The foregoing step c) is performed while the test
scanning is overlapped with the conversion operation.
[0521] While the foregoing steps a)-d) are an exemplary sequence of
making a conversion in units of image, scanning widths for faster
processing, the test can be conducted without fixing the conversion
unit to the image scanning width.
[0522] As an additional function to the operation for converting
vector data to raster data, at least one of the following functions
is provided.
[0523] a) A function of converting raster data to multi-value data.
b) A function of setting a gradation weight and an offset for the
conversion to multi-value data in view of the sensitivity of the
inspection apparatus in regard to the foregoing function a).
[0524] c) A function of processing an image for modifications such
as expansion, reduction and the like after vector data has been
converted to raster data.
[0525] Inspection steps based on the CAD data comparison method
executed in the apparatus illustrated in FIG. 42 are as
follows:
[0526] a) A step of converting CAD data to raster data in a
computer 1, and generating a reference image and saving the
reference image in the memory 248 with the aid of the foregoing
additional function.
[0527] b) A step of setting the switch 245 and switch 246 such that
at least one of the memory 241 and memory 242 of the image
processing apparatus is connected to the path 249 from the memory
248.
[0528] c) A step of transferring the reference image in the memory
248 to a memory of the image processing apparatus.
[0529] d) A step of comparing an image generated by sequentially
scanning the image under inspection, the image in the memory to
which the reference image has been transferred, and image data
which is at the same relative position in the die to find
differences.
[0530] e) A step of determining defects from the differences found
in the foregoing step d).
[0531] f) A step of subsequently testing or inspecting the same
portions for a scanning position of the reference die and the die
origin of the die under inspection over the entire wafer, as
illustrated in FIG. 44, and repeating the foregoing steps a) to e)
while changing the scanning position of the reference die until the
entire die is tested.
[0532] Settings are made to correct two images to be compared such
that a position alignment is made, i.e., a difference in position
is eliminated in the two images before the differences are found in
step d). Alternatively, a correction is made to eliminate density
alignment, i.e., a difference density. In some cases, both
processes may be required. The reference die image stored in the
memory of the image processing apparatus in step c) may be the
entire reference die, or a portion of the reference die which may
be tested while it is updated.
[0533] While the foregoing description has been made on the
algorithms of the array test (cell test) for inspecting periodic
structures, and the random test, the cell test and random test can
be conducted simultaneously. Specifically, the cell area and random
area are separately processed, wherein cells are compared with one
another in a die in the cell area, and simultaneously, a comparison
is made with adjacent dies, reference die, or CAD data-in the
random area. By doing so, the inspection time can be largely
reduced to improve the throughput.
[0534] In this event, inspection circuits for the cell area are
preferably provided independently of one another. Also, if tests
are not conducted simultaneously, a single inspection circuit may
be provided with programs which can be switched for the cell test
and random test, so that the comparison test can be conducted by
switching the programs. Specifically, when patterns are tested with
a plurality of processing algorithms applied thereto, these
algorithms may be executed simultaneously with separate circuits
provided therefor, or algorithms corresponding to them may be
provided and switched by a single circuit for processing. In any
case, this method can be applied as well when there is a plurality
of types of cells which are compared with one another, and dies are
compared with each other or with CAD data in the random
section.
[0535] FIG. 45 illustrates a basic flow of a focus function. First,
after transferring a wafer including an alignment operation,
recipes are created for setting conditions related to the test, and
the like. One of the recipes is a focus map recipe which is relied
on to perform an auto-focus operation during a inspection operation
and a reviewing operation in accordance with focus information set
therein. The following description will be made on a procedure of
creating the focus map recipe, and a procedure of the auto-focus
operation.
[0536] In the following example, the focus map recipe has an
independent input screen, and the operator executes the following
steps to create the focus recipe. Such an input screen may be added
to an input screen provided for different purposes.
[0537] a) A step of entering focus map coordinate representing the
position of a die, a pattern within the die, or the like for which
a focus value is entered. A switch 251 in FIG. 46.
[0538] b) A step of setting a die pattern which is required for
automatically measuring a focus value. This step may be skipped
when the focus value is not automatically measured.
[0539] c) A step of setting a best focus value at the coordinate on
the focus map determined at the foregoing step a).
[0540] Among the foregoing steps, while the operator can specify an
arbitrary die at step a), other setting can also be made, such as a
selection of all dies, a selection of every n die, and the like. In
addition, the operator can select the input screen from any of a
figure which schematically represents the arrangement of dies
within a wafer and an image which uses an actual image.
[0541] Among them, at step c), the operator manually selects and
sets in a mode manually set by a focus switch 252 associated with a
voltage value provided to a focusing electrode (switch 253 in FIG.
46). A mode for automatically finding a focus value to be supplied
(switch 254 in FIG. 46).
[0542] A procedure for automatically finding a focus value at the
forgoing step c) involves, for example, the following steps of:
[0543] a) finding an image with a focus position Z=1, and
calculating the contrast thereof;
[0544] b) performing the foregoing step a) while each of focus
positions Z=2, 3, and 4;
[0545] c) regressing from the contrast values calculated at steps
a) and b) to find a contrast function (see FIG. 48); and
[0546] d) calculating a Z value which results in a maximum value of
the contrast function, and choosing it to be the best focus
value.
[0547] For example, a die pattern required for automatically
measuring a focus value presents good results when a selected
pattern consists of alternating lines and spaces as illustrated in
FIG. 48, the contrast can be measured irrespective of the shape of
a black and white pattern, whichever one is selected.
[0548] The single best focus value can be found by executing steps
a) to d). A data format in this event is (X, Y, Z), which is a
combination of a set of the coordinate values X and Y at which the
focus is found, and the best focus value Z. Therefore, there exist
a number of focus map coordinates (X, Y, Z) determined by the focus
map recipe. This is part of the focus map recipe, and is called a
"focus map file."
[0549] A method of setting a focus to the best focus during a
inspection operation for capturing an image and a reviewing
operation, is implemented by the following steps.
[0550] a) Positional information is further sub-divided based on
the focus map file 1 created during the creation of the focus map
recipe, and the best focus at this time is calculated to create a
sub-divided focus map file 2.
[0551] b) The calculation at step a) is performed using an
interpolation function.
[0552] c) The interpolation function at step b) may be linear
interpolation, spline interpolation or the like, and is specified
by the operator upon creation of the focus map recipe.
[0553] d) The current X-Y position is monitored on the stage, and a
voltage at the focus electrode is changed to a focus value
described in the focus map file 2 suited to the current X-Y
position.
[0554] Describing more specifically with reference to FIG. 49, a
black circle represents a focus value of the focus map file 1, and
a white circle represents a focus value of the focus map file 2,
wherein:
[0555] 1. focus values of the focus map file 2 are inserted between
focus values of the focus map file 1; and
[0556] 2. a focused position Z is varied following the scanning to
maintain the best focus. In this event, the value of the preceding
focus value is maintained between two white circles until the focus
position is varied next time.
[0557] FIG. 50 illustrates an exemplary semiconductor manufacturing
plant which employs the electron beam apparatus according to the
present invention. In FIG. 50, the electron beam apparatus is
designated by a reference numeral 261. Information such as a lot
number of wafers to be tested by the electron beam apparatus,
histories of manufacturing apparatuses, and the like are read from
a memory included in SMIF or FOUP 262, or the lot number can be
recognized by reading an ID number of the SMIF, FOUP 262 or a wafer
cassette. During the transfer of wafers, the amount of moisture is
controlled to prevent oxidization of metal wires and the like.
[0558] A PC 263 of a defect detector 261 for controlling a defect
detection is connected to an information communication network 264
of a production line, so that information such as a lot number of
wafers which are objects under inspection, and the result of their
tests can be sent to a production line control computer 265, a
variety of manufacturing apparatuses 266, and other inspection
systems through the network 264. The manufacturing apparatuses 266
include those associated with lithography, for example, an exposure
apparatus, a coater, a curing apparatus, a developer, and the like,
an etching apparatus, deposition apparatuses such as a sputtering
apparatus and a CVD apparatus, a CMP apparatus, a variety of
measuring apparatuses, other inspection apparatus, and the
like.
[0559] FIG. 51 is a diagram generally illustrating a fourth
embodiment of the electron beam apparatus according to the present
invention. The present invention will be described with reference
to this figure.
[0560] An electron gun 271 comprises a cathode made of lanthanum
hexaboride (hereinafter called "LaB6") having electron emission
capabilities; a Wehnelt electrode having an electrode perpendicular
to the optical axis; and an anode. By operating the cathode in a
space charge control region, shot noise can be reduced during
use.
[0561] Electron beams emitted from the electron gun are converged
by a condenser lens 272 which comprises an electromagnetic lens, to
form a cross-over at a point closer to the electron gun than
formation apertures 273. Electron beams diverged from the
cross-over can irradiate the formation apertures 273 with a uniform
intensity. When the irradiation intensity is too low, the
cross-over image can be brought closer to the formation apertures
273, thereby increasing the irradiation intensity. On the other
hand, when a uniform irradiation intensity is provided in a small
region, the cross-over position may be brought closer to the
electron gun.
[0562] Electron beams formed into a rectangular shape by the action
of the formation apertures 273, are reduced in size by a condenser
lens 273 comprising an electromagnetic lens, and an objective lens
279, and focused on the surface of a sample 282 to form a formation
image. Under the condenser lens 274, a deflector 275 is provided
for adjusting the trajectory of the primary beams.
[0563] The cross-over image formed in front of the formation
apertures 273 is focused on the main surface of the objective lens
279 by the condenser lens 274 to ensure the separation of the
formation image from the cross-over image. A plurality of formation
apertures 273 are provided in order to replace one with another
when the one gets dirty, and vary a irradiated range in accordance
with a change in pixel dimensions.
[0564] When the formation apertures are changed in dimensions, the
excitation of the condenser lens 274 is also changed. For example,
with a small formation aperture, the cross-over image is brought
closer to the formation aperture 273 to increase the current
density. The adjustment thus made causes the cross-over image to
deviate from the main surface of the objective lens 279. However,
with the employment of electromagnetic lenses for the condenser
lenses 272 and 274 which corrects this deviation by changing the
excitation of the lens 274, the supply current need not be largely
changed when the energy of primary electron beams is changed, so
that a smaller burden is imposed than when electrostatic lenses are
employed.
[0565] Electromagnetic deflectors 175 are provided at two stages
behind the condenser lens 274. As a result, primary electron beams
can be adjusted to pass along a shifted trajectory, rather than
passing along the same passage as secondary electron beams below an
ExB separator which comprises an ExB electrostatic deflector 276,
an ExB electromagnetic deflection coil 277, and an ExB deflection
core 278.
[0566] Secondary electrons from the sample are deflected by .alpha.
by the electrostatic deflector 276 of the ExB separator by
-2.alpha. and by an electromagnetic deflector. In this event, since
the amount of electrostatic deflection is one-half as much as the
amount of electromagnetic deflection and is reverse in direction to
the same, the resulting design can substantially eliminate
deflection chromatic aberration which is the major aberration of
the ExB separator. Since the primary electrons are slightly higher
in energy than the secondary electrons, the primary electrons are
deflected by 2.8.alpha. to the left and impinge on the sample 282.
It should be noted that in the figure, the deflection to the left
is defined as positive.
[0567] It is important that a Bohr radius 283 (inner diameter of a
cylindrical portion at the center of the objective lens) of the
objective lens, and the distance 284 between the sample and the
main surface of the objective lens are made larger than the Bohr
radius of the objective lens. As a result, the secondary electrons
emitted from the sample in the normal can direction intersect with
the optical axis and pass through the NA aperture. Even with an
objective lens which has a lens gap closer to a sample, a similar
effect can also be produced by increasing the distance between the
sample and the bottom of the objective lens magnetic pole beyond
the Bohr radius of the lens.
[0568] A lens gap 280 of the objective lens 279 is as small as 2 mm
or less, while the Bohr radius is as large as 20 mm.phi.. In
addition, the objective lens 279 is a normal magnetic lens, i.e., a
lens which has the magnetic gap closer to the optical axis. In this
event, the Bohr radius is chosen to be 20 mm from the fact that the
visual field extends over 200 .mu.m, but by setting the Bohr radius
to 80 times or more of the maximum diameter of the visual field,
the secondary electrons or reflected electrons emitted from the
sample in the normal direction can intersect with the optical axis
and pass through the NA aperture. Note, however, that the diameter
of the visual field is defined herein to be the length of the
diagonal of the image on the surface of the aforementioned
rectangular sample.
[0569] A Z-direction dimension from the sample 282 to the center of
the lens gap 280 is made larger than the Bohr radius so as not to
increase chromatic aberration of higher-order magnification and
rotation (blur proportional to R3.DELTA.V/V). Also, an axially
symmetric cylinder electrode 281 is provided near the lens gap 280,
and by applying a positive high voltage to this cylindrical
electrode 281, the axial chromatic aberration of the objective lens
can be reduced between this cylindrical electrode, which can reduce
the axial chromatic aberration and the sample, without ensuring a
distance enough to avoid a discharge, and without causing a
discharge between the objective lenses.
[0570] The secondary electron or reflected electron image is
enlarged approximately by a factor of five by the objective lens
279, and further enlarged by a factor of five and by a factor of
ten by the electromagnetic lenses 285 and 286, respectively, i.e.,
totally enlarged by a factor of 250, and creates an enlarged image
on a scintillator 287 which is adjusted in magnification by an
optical lens system, detected and converted into an electric signal
by a TDI detector or a CCD detector, and processed into
two-dimensional image data by a signal processing circuit.
[0571] Electrodes 288, 289, 290, 291 make up an electrostatic lens,
and acts as an auxiliary lens for creating a cross-over image
produced by the objective lens substantially on the main surface of
the lens 285. This lens can reduce the beam bunch and aberration at
the position of the lens 285. Also, the image of the sample by the
objective lens 279 is formed on the main surface of this
electrostatic lens. This electrostatic lens does not affect the
magnification. Likewise, a sample image produced by the lens 285 is
formed on the main surface of the auxiliary lens 292, and is
focused on the surface of the scintillator 287 by the lens 286. The
cross-over image produced on the main surface of the lens 285 is
formed on the main surface of the lens 286 by the auxiliary lens
292, and the lens 286 reduces the beam bunch and aberration.
[0572] By switching the formation of image between a
high-throughput low-accuracy mode and a low-throughput
high-accuracy mode, the irradiated area and magnification need be
varied. The former may simply involve switching the formation
aperture 273 to one having dimensions compatible with the mode. The
latter, i.e., variations of the magnification may involve applying
a lens voltage to the electrode 290, grounding the electrodes 288,
289, 291, reducing the excitation of the lens 279, and matching a
first enlarged image with the position of the electrode 290, thus
resulting in a higher magnification. A low-magnification image can
be produced by applying a lens voltage to the electrode 289, and
matching the first enlarged image with the position of the
electrode 289. Since the electrodes 288, 291 are grounded at all
times, they need not be insulated.
[0573] An NA aperture 293 of the secondary optical system is
provided on the main surface of the electromagnetic lens 285. Since
the lenses 272, 274, 286, 292 may be electrostatic or
electromagnetic, because they are not related to aberration.
However, with beam energy of approximately 4 KeV, the
electromagnetic lens more easily reduces the focal distance, and
can reduce the focal distance of the lens with a practical driving
power supply, so that it can reduce the length of the barrel** and
a blurred beam due to the space charge effect.
[0574] FIG. 52 is a diagram illustrating an electro-optical system
in a fifth embodiment of the electron beam apparatus according to
the present invention.
[0575] An electron gum comprises a cathode 301 made of LaB6; a
Wehnelt electrode 302 having an electrode perpendicular to the
optical axis; and an anode 303. By operating the cathode in a space
charge control region, shot noise is reduced by a factor of five as
compared with a Schottky cathode and an FE gun.
[0576] Electron beams emitted from the electron gun are aligned by
an alignment deflector 304 and enters a condenser lens 305 which
comprises an electromagnetic lens. Electron beams converged by the
condenser lens 305 forms a cross-over at a position closer to the
electron gun than a formation aperture 308. Electron beams diverged
from the cross-over can irradiate the formation apertures 308 with
a uniform intensity. Electron beams formed into a rectangular shape
by the action of the formation apertures 308, are reduced in size
by a condenser lens 309 and an objective lens 314, and forms a
formation image on the surface of the sample 313. In this event,
the cross-over image created in front of the formation apertures
308 is focused on the objective lens 314 by the formation lens 309
to ensure the separation of the formation image from the cross-over
image.
[0577] A plurality of formation apertures 308 are provided in order
to replace one with another when the one gets dirty, and vary a
irradiated range in accordance with a change in pixel dimensions.
When the formation apertures are changed in dimensions, the
excitation of the condenser lens is changed. For example, with a
small formation aperture, the cross-over image is brought closer to
the formation aperture 273 to increase the current density. The
adjustment thus made causes the cross-over image to deviate from
the main surface of the objective lens 279, however, without
causing any grave problem.
[0578] While the lens excitation condition must be changed in order
to change the energy of the primary electron beams, with the
employment of electromagnetic lenses for the condenser lenses 305
and formation lens 309, the supply current need not be largely
changed when the energy of primary electron beams is changed, so
that a smaller burden is imposed than when electrostatic lenses are
employed.
[0579] Deflectors 310, 311 are provided at two stages below the
formation lens 309, such that the primary beams of electron beams
pass along a trajectory deviated from the main beam of the
secondary electrons or reflected electron beams, as indicated by
312. The alignment to the formation apertures 308 and formation
lens 309 is performed using the electrostatic deflectors 306, 307
at two stages, so that the two electrostatic deflectors can be set
at two different deflection ratios, specifically, (1) a setting
which causes deflection fulcrums of the two deflectors to match
with formation aperture 308, and (2) a setting which causes the
same to match which the main surface of the formation lens 309. By
selecting one of the settings, the alignment can be performed
without exerting influences on each other. The objective lens 341
will be described with reference to FIG. 55.
[0580] Now, a description will be made of two scenarios, where the
visual field is divided into sub-visual fields, in each of which a
sample image is captured, and where a sample image is collectively
captured without deflectors. When a sample image is captured
without deflectors, it is necessary to solve a problem that
secondary electrons or reflected electrons emitted from a visual
field end in the normal direction do not pass through the NA
aperture. With the employment of a magnetic lens which has a
smaller Bohr radius on the sample side for an objective lens, the
secondary electrons or reflected electrons can pass through the NA
aperture, and the secondary electrons or reflected electrons can be
improved in transmittance. In the present invention, the Bohr
radius of the lower magnetic pole is smaller than the Bohr radius
of the upper magnetic pole. Ideally, the Bohr radius of the upper
magnetic pole should be 1.5 times as large as the Bohr radius of
the lower magnetic pole. As a result of a simulation in which the
Bohr radius of the lower magnetic pole was chosen to be 8 mm, and
the distance between a sample and the main surface of the objective
lens was chosen to be 10 mm, it has been found that secondary
electrons emitted in a direction inclined by eight degrees with
respect to the normal of the surface of the sample intersect with
the optical axis. Accordingly, from the cosine rule, components of
the secondary electrons centered at eight degrees will pass.
[0581] On the other hand, in a division image capture which divides
the visual field into sub-visual fields, in each of which a sample
image is captured, the visual field is divided into 4-20 sub-visual
fields, where electron beams can be corrected for aberrations using
deflectors and the like to capture a sample image in a low
aberration condition. In this event, the visual fields should be in
the shape of a square or a rectangle which is similar to a square
and is slightly shorter in the longitudinal direction of the visual
field.
[0582] A MOL method is known in the field of lithography as an
approach for causing electron beams to pass away from the optical
axis of a lens, thereby preventing the electron beams from being
affected by aberration.
[0583] Now, the objective lens will be described with reference to
FIG. 55. An upper deflector 342 and a lower deflector 341 are
disposed across a lens magnetic field 344, such that a lower
deflection magnetic field is generated in an orientation 346 for
electron beams which pass to the right of the optical axes, and the
upper deflector produces a deflection magnetic field in an
orientation 347. Quantitatively, a resulting deflection magnetic
field is proportional to a differentiated value in regard to Z of
an axial magnetic field distribution of the objective lens 341. The
axial magnetic strength distribution of the objective lens 314
presents a maximum value at the position of the objective lens
magnetic field 344, positive values in a region above this
position, and negative values in a region below this position in
regard to Z. The visual field can be expanded with reduced axial
chromatic aberration by using a magnetic lens for the objective
lens, providing electromagnetic coils at two stages before and
after the main surface of the objective lens thereacross, and
designing these deflectors to substantially satisfy the MOL
condition.
[0584] Accordingly, the deflection coil which satisfies the MOL
condition may comprise two deflection coils 341, 342 disposed above
and below the lens main surface, as illustrated in FIG. 55, where
the upper deflection coil 342 may present a small peak value and a
large half-value width, while the lower deflection coil may present
a large peak value and a small half-value width.
[0585] A ferromagnetic material 343 outside the deflection coil is
required to respond well at high frequencies with a ferrite adhered
at least to the surface thereof. Also, a liner tube (vacuum wall)
315 may be made of an insulating material such as ceramics or the
like, the surface of which is coated with a conductive material.
The deflection coil 341, which must be contained in vacuum may be
contained within a coaxially symmetric electrode 345, the surface
of which is coated with a metal. In FIG. 55, as a high negative
voltage is applied to the coated surface of the axially symmetric
electrode 345, the axial chromatic aberration can be advantageously
reduced.
[0586] Secondary electrons or reflected electrons from a sample
pass through the objective lens, and area deflected by an ExB
separator (which comprises the electrostatic deflector 320 of the
ExB separator and the electromagnetic deflector 321 of the ExB
separator). The optical axis of the secondary optical system is
present in a direction inclined by 10 degrees to the right on the
figure. The secondary electrons or reflected electrons are
deflected by ten degrees to the left by the electrostatic deflector
320, and receives a force from the electromagnetic deflector 321
which deflects the electrons by 20 degrees to the right, and are
therefore deflected by 10 degrees to the right in total. Here,
since the electrostatic deflection amount is one-half as much as
the electromagnetic deflection amount and is reverse in direction
to the same, the resulting design can substantially eliminate
deflection chromatic aberration which is the major aberration of
the ExB separator.
[0587] An auxiliary lens 326 acts to converge a cross-over image
formed by the secondary electrons or reflected electrons emitted
substantially perpendicular from the sample slightly in front of
the ExB separator to focus the cross-over image on the main surface
of the magnification lens 327.
[0588] Three types of enlarged images can be provided by selecting
which of three electrodes of the auxiliary lens 326 for the sample
image produced by the objective lens 314. For example, when the
position of the sample image is formed at the second lowest
electrode from below, the resulting magnification is the smallest.
In this event, a voltage from a power supply 325 for driving the
electrostatic lens is applied from this electrode, while the other
electrodes are all grounded through a switch 324. This first image
is enlarged by the magnification lens 327 to create an enlarged
image at the position of the second electrode of the magnification
lens 338 at the final stage, which is further enlarged by a lens
action with a negative voltage applied from the fourth electrode
from below to create a final image on the surface of a scintillator
coated on a flat region of a ball lens 335 made of melted
quartz.
[0589] The main surface of the magnification lens 327 has an NA
aperture 328 and determines a compromise between aberration and
secondary electron transmittance. The NA aperture 328 has a
dimension of several tens to one hundred .mu.m. The aberration of
the auxiliary lens 326 does not at all cause any problem. This is
because the position of the electrode serves as an image plane for
the sample image so that no lens action develops and no aberration
is produced. Accordingly, the power supply 325 supplies a negative
voltage. Since the image is focused with a negative voltage of
several kV, a simple insulating structure may be employed.
[0590] A lens at the final stage may be an electrostatic lens
having five electrodes, with a central electrode thereof being
applied with a voltage different in sign from voltages applied to
electrodes before and after the central electrode, resulting in a
maximum magnification. In some cases, the optical distance between
the lenses can be shorter than the mechanical dimension, which can
be achieved under a low aberration condition. Consequently, the
secondary electron or reflected electron image can be improved in
magnification.
[0591] For achieving the maximum magnification, the fourth
electrode from below is applied with a voltage from the switch 323
of the power supply, causing the sample image to be focused at the
position of this electrode. In this event, all the other electrodes
are grounded. This results in a larger image point of the objective
lens and a smaller object point of the magnification lens 327 to
provide the maximum magnification.
[0592] In designing the final lens 300, the following two aspects
should be taken into consideration.
[0593] (1) The Bohr radium of the electrode at the position of the
auxiliary lens should be sufficiently larger than the diameter of a
second image at this position; and (2) the distance between the
main surface of the auxiliary lens and the main surface of the
magnification lens should be reduced in order not to increase so
much the enlarged image at this position and instead to increase
the magnification. Of course, the focal distance should be made
short in correspondence to this distance between the main surfaces
under the foregoing conditions.
[0594] For reducing the focal distance of the auxiliary lens and
the focal distance of the magnification lens, the present invention
applies the third electrode with a voltage 331 which has a reverse
sign to voltages applied to the second and fourth electrodes. By
doing so, the electric field intensity between the second and third
electrodes increases to produce a large lens action, as compared
with a normal configuration in which the electrode is grounded.
Another advantage is that the optical distance between the two
lenses is reduced because the main surfaces of the lenses move
toward the electrode simultaneously applied with a positive voltage
due to a higher lens action of this electrode.
[0595] Also, when the aberration is compared with the magnification
lens being driven with a negative voltage and with a positive
voltage, no significant difference is found between the two cases.
When an electromagnetic lens is employed for the magnification
lens, distortion aberration is larger than when an electrostatic
lens is employed. However, since the lens 327 is required to have
the NA aperture disposed on the main surface, an electromagnetic
lens should be employed therefor. Since this lens produces a small
image, the value of distortion can be negligible (equal to or less
than one tenth of the pixel) though the distortion aberration
coefficient is large.
[0596] The alignment of the auxiliary lens 326, which is an
electrostatic lens, with the electromagnetic lens 327 is performed
by the ExB separator and alignment deflector 322 by matching the
center of deflection with the auxiliary lens 326 or with the
electromagnetic lens 327. The alignment of the magnification lens
330 to a bear is performed by an alignment deflector 329. Alignment
334 is intended for a scintillator 336.
[0597] The overall visual field can be reduced in aberration by
making adjustments to correct field curvature aberration and
anastigmatic, and reduce the difference in beam resolution between
the center and periphery of the visual field. A ball lens 335 is
designed such that the distance between the center of the sphere
and a plane is 1/n (n is the refractive index of the material, and
is 2.1 for melted quartz) of the radius of the sphere. Under this
condition, a so-called aplanatic hyper-hemisphere is established,
where, advantageously, there is no spherical aberration, or
anastigmatic, or chromatic aberration in the axial direction, the
light emission direction is narrowed down to 1/n, and apparent
dimensions of the scintillator, viewed from the optical lens 338 is
increased by a factor of n.sup.2. When the ball lens is corrected
for other chromatic aberration and distortion by the optical lens
338, the optical lens system can be increased in transmittance even
if the optical lens 338 has a large f-number. Even if the lens
exhibits a low resolution, this does not cause any problem because
the actual scintillator image is increased by a factor of n.sup.2.
Also, since the optical lens 338 has a large f-number, a simple
lens may be employed therefor.
[0598] The electron beam apparatus having the electron-beam based
inspection function can be applied to evaluate wafers during a
semiconductor device manufacturing process. An exemplary
semiconductor device manufacturing process will be described below
with reference to a flow chart of FIG. 53.
[0599] (1) a wafer manufacturing process for preparing a wafer (or
a wafer preparing process for preparing a wafer) (step 401);
[0600] (2) a mask manufacturing process for manufacturing masks for
use in exposure (or mask preparing process for preparing
masks)(step 402);
[0601] (3) a wafer processing process for performing processing
required to the wafer (step 402);
[0602] (4) a chip assembling process for cutting one by one chips
formed on the wafer and making them operable (step 403);
[0603] (5) a chip testing process for testing complete chips (step
404);
[0604] (6) a process for repeating the processes (2) and (3) as
required; and
[0605] (7) cutting the wafer and assembling devices.
[0606] The respective main processes are further comprised of
several sub-processes. Among these main processes, the wafer
fabricating process exerts critical affections to the performance
of resulting semiconductor devices. This process involves
sequentially laminating designed circuit patterns on the wafer to
form a large number of chips which operate as memories, MPUs and so
on. The wafer fabricating process includes the following
sub-processes:
[0607] (A) a thin film forming sub-process for forming dielectric
thin films serving as insulating layers, metal thin films for
forming wires or electrodes, and so on (using CVD, sputtering and
so on);
[0608] (B) an oxidation sub-process for oxidizing the thin film
layers and the wafer substrate;
[0609] (C) a lithography sub-process for forming a resist pattern
using masks (reticles) for selectively fabricating the thin film
layers and the wafer substrate;
[0610] (D) an etching sub-process for fabricating the thin film
layers and the substrate in conformity to the resist pattern
(using, for example, dry etching techniques);
[0611] (E) an ion/impurity implantation/diffusion sub-process;
[0612] (F) a resist striping sub-process; and
[0613] (G) a sub-process for testing the fabricated wafer.
[0614] As appreciated, the wafer fabrication process is repeated a
number of times equal to the number of required layers to
manufacture semiconductor devices which operate as designed.
[0615] FIG. 54 is a flow chart illustrating the lithography
sub-process which forms the core of the wafer processing process.
The lithography sub-process includes the following steps:
[0616] (a) a resist coating step for coating a resist on the wafer
on which circuit patterns have been formed in the previous process
(step 405);
[0617] (b) a resist exposing step (step 406);
[0618] (c) a developing step for developing the exposed resist to
produce a resist pattern (step 407); and
[0619] (d) an annealing step for stabilizing the developed resist
pattern (step 408).
[0620] Known processes are applied to the foregoing semiconductor
device manufacturing process, wafer fabrication process and
lithography process. When the electron beam apparatus according to
the aforementioned embodiment of the present invention is employed
in the wafer testing sub-process (G), the transmittance of
secondary electron beams can be improved without causing a
discharge between a sample and the objective lens to enable an
efficient and highly accurate inspection, thus making it possible
to increase the yield rate of products.
[0621] FIG. 56 is an explanatory diagram illustrating an
electro-optical system of a sample evaluation apparatus such as a
defect inspection apparatus which is a sixth embodiment of the
electron beam apparatus according to the present invention. As
illustrated in FIG. 56, in the sample evaluation apparatus of the
present invention, electron beams emitted from an electron gun 501
are converged by a condenser lens 502, and formed into a
rectangular shape through a rectangular aperture 503, such as
square. The resulting rectangular electron beams are irradiated to
the surface of a sample 506 through an irradiation lens 504 and an
objective lens 505. In this event, the rectangular electron beams
are deflected by an ExB separator 509 disposed within the objective
lens 505, such that the electron beams are irradiated
perpendicularly to the surface of the sample. Also, the rectangular
electron beams are deflected by deflectors 519 and 520 such that
the rectangular electron beams move within the visual field.
[0622] On the other hand, secondary electrons emitted from the
sample 506 by this irradiation are converged by the objective lens
505 to form an enlarged image near an alignment deflector 511. The
image is corrected for chromatic aberration by a chromatic
aberration corrector 512, and formed in front of a magnification
lens 513. In this embodiment, the chromatic aberration corrector
512 comprises quadrupole lenses stacked one on another at four
stages. Then, the magnification lens 513 forms an enlarged image in
front of a magnification lens 514, and the magnification lens 514
forms an enlarged image on the surface of a scintillator 516. In
this event, a deflector 515 deflects the enlarged image such that
it is formed on a corresponding detection plane of the scintillator
516 in synchronism with movements of the rectangular electron beams
within the visual field by deflectors 519 and 520.
[0623] The scintillator 516 has a plurality of detection plane
arranged in matrix, each of which corresponds to a sub-visual
field. When primary electron beams are formed into a square shape,
the scintillator 516 comprises 16 detection planes arranged, for
example, in four rows and four columns. Image data detected by each
detection plane of the scintillator 516 is transferred to CCD
detectors (or MOS image sensors) associated with 512.times.512
pixels for conversion into an electric signal.
[0624] The objective lens 505 contains a MOL (Moving Objection
lens) 7 and an axially symmetric electrode 508. While electron
beams present larger aberration as they are further away from the
optical axis, they can move in parallel with the axis of the
objective lens by an appropriate electric field or magnetic field
applied by these deflector and axially symmetric electrode, thus
making it possible to provide a wide visual field. The MOL
deflector 507 generates a magnetic field for reducing aberration of
secondary electrons from the visual field other than the optical
axis. Here, since the secondary electrons emitted from the sample
506 have an energy width, large axial chromatic aberration occurs
after the secondary electron beam has passed through the objective
lens. This axial chromatic aberration is corrected by axial
chromatic aberration caused by the chromatic aberration corrector
512. A voltage applied to the axially symmetric electrode 508 is
adjusted such that negative chromatic aberration by the chromatic
aberration corrector 512 is equal to positive chromatic aberration
caused by the objective lens 505 in absolute value.
[0625] In the present invention, aberration is reduced near the
optical axis by thus correcting the axial chromatic aberration.
Also, since off-axis aberration is corrected, CMOS image sensors
are made available by dividing the visual field into a plurality of
sub-fields and using the MOL deflector, so that the evaluation can
be made at a high throughput.
[0626] In this regard, the objective lens 505 is configured to have
a Bohr radius D which is larger than the diameter of the visual
field by a factor of 50 or more, and a magnetic gap 518 is defined
near the sample 506 for reducing the axial chromatic
aberration.
[0627] FIGS. 57(A) and 57(B) are explanatory diagrams illustrating
an electro-optical system of an electron beam drawing apparatus
which is a seventh embodiment of the electron beam apparatus
according to the present invention. In this apparatus, as
illustrated in FIG. 57(A), electron beams emitted from an electron
gun 531 are adjusted by two condenser lenses 532 and 533 for a
uniformly irradiated region, and are irradiated to a rectangular
aperture 534 in the shape of square or the like. Rectangular
electron beams formed by the rectangular aperture are focused on a
character mask 536 by a formation lens 535. The character mask 536
is provided with a plurality of magnification transmission masks
for circuit patterns which should be transferred onto desired dies.
By deflecting the rectangular electron beams by the deflector 546,
a circuit pattern mask is selected on the character mask 536, and
by deflecting the rectangular electron beams by the deflector 547,
the rectangular electron beams are returned to the original optical
axis.
[0628] The patterned rectangular electron beams are scaled down by
a formation lens 537 and a reducing lens 538 to form a reduced
image of the selected circuit pattern in front of a chromatic
aberration corrector 539. Then, a reduced image is formed on a
sample 545 through an objective lens 540 which comprises an
electromagnetic lens. The objective lens 540 contains deflectors
541-544 for moving a circuit pattern image to a position within the
visual field, at which a drawing is desired, and for reducing
deflection aberration. It should be noted that the off-axis
aberration can be reduced by a combination of electromagnetic
deflectors 431-544 which are contained in the objective lens
540.
[0629] In this way, according to the electron beam drawing
apparatus illustrated in FIG. 57, the axial chromatic aberration
can be corrected by the chromatic aberration corrector 539, while
the off-axis aberration can be corrected by the objective lens 540
and electromagnetic deflectors 541-544. Consequently, the aperture
angle of an NA aperture 550 can be increased to reduce the space
charge effect. Thus, since a drawing can be made with a large beam
current, an LSI pattern and the like can be drawn on the sample at
a high throughput.
[0630] Preferably, the reduced image formed by the reducing lens
538 is formed in front of the chromatic aberration corrector 539,
while the image formed by the chromatic aberration corrector 539 is
formed at a position 549 at which the reduction ratio by the
objective lens 540 is approximately one.
[0631] Also, the NA aperture 550 can be in an annular shape as
illustrated in FIG. 57(B) instead of a circular hole as illustrated
in FIG. 57(A). Conventionally, an annular aperture is associated
with a large aperture angle and large axial chromatic aberration
caused thereby, so that the annular aperture has not been used in
the past. The present invention can employ an annular NA aperture
having a large aperture angle by correcting the axial chromatic
aberration by the corrector, and as a result, can reduce the space
charge effect and use a large beam current.
[0632] FIG. 58 illustrates a sample evaluation apparatus which is
an eighth embodiment of the electron beam apparatus according to
the present invention. In this apparatus, electron beams emitted
from an electron gun 561 are converged by a condenser lens 562 to
form a cross-over in front of a multi-aperture pate 563 which is
provided with a plurality of apertures. Then, the electron beams
diverged from the cross-over are irradiated to the multiple
apertures to from a reduced image at a point P by lenses 564 and
565 which can be adjusted in reduction ratio and rotation angle,
and the reduced image is formed on a sample 574 through a chromatic
aberration corrector 568 and an objective lens 571 which is an
electromagnetic lens. In this event, the electron beams are scanned
on the sample 574 by electrostatic deflectors 569 and 575. For
reducing chromatic aberration when the electron beams are scanned
on a position away from the optical axis of the visual field, the
Bohr radius D of the objective lens 57 is preferably set to 50
times or more larger than the visual field.
[0633] In this eighth embodiment, the electron gun 561 is
preferably made of LaB.sub.6. Also, for scanning a wider visual
field, electromagnetic deflectors 572 and 573 are provided. These
electromagnetic deflectors 572 and 573 satisfy the MOL condition.
Specifically, an axial magnetic field distribution of the objective
lens 571 is represented by a function approximated to a Gaussian
distribution which has apeak at the center of a magnetic gap.
Therefore, observing its differentiation in the z-axis direction,
i.e., optical axis direction, i.e., a changing amount dz, this is
represented by a function which reaches zero at the center of the
magnetic gap and has opposite signs above and below the center. The
deflection magnetic field generated by the deflectors 572 and 573
may be adjusted so as to be proportional to the differentiation
function. However, a ferrite pipe 577 must be adhered to the inner
surface of the core of the objective lens 571.
[0634] Multiple secondary electron beams emitted from the sample
574 are accelerated by an acceleration electric field generated by
an axially symmetric electrode 576 applied with a positive voltage
and the surface of the sample 574 applied with a negative voltage,
and passes through the objective lens 571. Then, immediately before
passing through the objective lens 571, the secondary electron
beams are deflected in a direction (in the right-hand direction in
the figure) orthogonal to a scanning direction by a beam separator
570 disposed at an upper end within the objective lens, and an
enlarged image is focused on the surface of an FOP (fiber optical
plate) plate 581 on which the scintillator is coated. The FOP plate
581 is not a simple plate but optical fibers which comprise the FOP
are connected to PMTs 582 in a one-to-one correspondence, so that
the enlarged image is converted into an optical signal by the PMTs
582.
[0635] In this way, since the multiple secondary electron beams
emitted from the sample are input to the PMTs 582 in a one-to-one
correspondence, the problem of cross-talk can be avoided. Also,
when a periodic line and space pattern is formed on the sample, a
signal generated from each PMT 582 presents a periodic waveform
which repeats a high intensity and a low intensity, as indicated by
reference numerals 583-585. The PMTs 582 and associated amplifiers
are adjusted, while observing diameters 583-585 of the periodic
waveform, such that their contrast and offset values are
substantially the same.
[0636] In the embodiment illustrated in FIG. 58, in regard to
deflection chromatic aberration caused by the beam separator 570
for the primary electron beams, no deflection chromatic aberration
occurs in the primary electron beams because deflection aberration
caused by the electrostatic deflector 569 and electromagnetic
deflector 570 are canceled out by each other by setting the
distance D1 between an image formed by the chromatic aberration
corrector 568 and the electrostatic deflector 569 to be equal to
the distance D2 between the electrostatic deflector 569 and
electromagnetic deflector 570. Also, by setting a secondary
electron beam to be focused near the electromagnetic deflector 570,
deflection chromatic aberration to the secondary electron image can
also be reduced.
[0637] Field curvature which occur when scanning is performed at a
position far away from the optical axis can be corrected by
adjusting voltages applied to the axially symmetric electrodes 566
and 567 within the rotary lenses 564 and 565. The rotation of
electron beam can also be dynamically corrected by changing
voltages applied to the axially symmetric electrodes 566 and 567.
In this regard, the rotary lenses 564 and 565 are known lenses with
axial magnetic fields in directions opposite to each other. The
chromatic aberration corrector 568 comprises quadrupole lenses at
four stages which are arranged so as not to cause two-time
symmetric aberration, four-time symmetric aberration, or coma
aberration. A voltage applied to the electrode 576 is adjusted to
eliminate chromatic aberration while observing beam aberration.
[0638] Referring now to FIG. 59, a description will be given of a
method of capturing an image of the surface of a sample in the
sample evaluation apparatus which uses multiple beams, as
illustrated in FIG. 58. FIG. 59 schematically illustrates the
surface of a sample, where the stage is continuously moved in the
y-axis direction of the x-y coordinate system, and electron beams
are scanned in the x-axis direction. Assume also that in this
example, multiple beams are formed in six rows and five
columns.
[0639] When multiple beams are arranged in six rows and five
columns, the rows of the multiple beams may be inclined by
sin.sup.-1(1/5) with respect to the y-coordinate (therefore, the
columns to the x-axis) to equal a raster pitch during multi-beam
scanning. In this regard, while the raster pitch may be set to an
integer multiple of pixels, but in order to use a largest possible
number of multiple beams within a fixed distance from the optical
axis, the raster pitch is preferably set to be equal to the pixel
dimensions. With n rows and m columns (rows are in close proximity
to the y-axis direction, while columns are in close proximity to
the x-axis direction), m n is preferred, where the beam interval is
calculated by n*pixel dimension/cos[sin.sup.-1(1/n)]. When the
beams are arranged in an orthogonal matrix, the most beams can be
populated in a unit area, however, the beams do not necessarily
have to be orthogonally arranged when they are arranged to be at
the same raster pitch during scanning.
[0640] When raster scanning is performed as illustrated in FIG. 59
for a cell-by-cell inspection, signals at the same positions 597,
598, 599 (or 600, 601, 602) within a cell found by the same
scanning of the same beam are compared with each other for
evaluation such as a defect inspection or the like.
[0641] For conducting a die-by-die inspection, signals at the same
y-axis position on different dies may be compared with each other
for evaluation. Signals from the same electron beam at the same
y-coordinate on different dies may be compared with each other.
[0642] Preferably, two-dimensional patterns are created for one
scanning session or for one cell, such that the comparison and
evaluation are performed with a two-dimensional pattern in the
scanning direction, i.e., in the x-axis direction in a cell-by-cell
inspection, while die data are compared with each other in the
stage moving direction, i.e., in the y-axis direction for
evaluation.
[0643] FIG. 60 is an explanatory diagram illustrating an
electro-optical system of a sample evaluation apparatus which is a
ninth embodiment of the electron beam apparatus according to the
present invention. In this apparatus, electron beams emitted from
an electron gun 611 are converged by a condenser lens 612, formed
into rectangular electron beam by a rectangular aperture formed
through an aperture plate 613 in a square shape or the like, and
irradiated to a sample 618 through a formation lens 614 and an
objective lens 617 which is a unit-potential lens.
[0644] Secondary electrons emitted from the sample 618 by the
irradiation are accelerated by the objective lens 617, and
separated from primary electron beams by a beam separator 616.
Then, the secondary electrons are deflected in the vertical
direction by an electrostatic deflector 619 and focused at a
position 621 in front of a chromatic aberration corrector 620. An
image of the chromatic aberration corrector 620 is enlarged by
magnification lenses 622, 623 at two stages to focus an image on a
detection plane of a detector 624.
[0645] Here, the beam separator 616 is a pure electromagnetic
deflector, and is set to provide the same deflection amount as the
deflection amount of the electrostatic deflector 619 for deflecting
the secondary electron beams. In addition, the distance between the
position 621 and deflector 619 is set to be equal to the distance
between the electrostatic deflector 619 and beam separator 616,
thereby eliminating deflection chromatic aberration. Also, even
when the objective lens 617 comprises a single lens, off-axis
aberration can be reduced to a negligible degree by optimizing the
position of an NA aperture 625. Further, the beam separator 616 can
be an ExB separator similar to that in the embodiment illustrated
in FIG. 56, and the deflector 619 can be an electrostatic
deflector.
[0646] When the rectangular electron beams are moved within the
visual field, field curvature can be corrected by adjusting a
voltage applied from a voltage source 626 to an upper electrode or
a lower electrode of the objective lens 617. Since the upper
electrode is at a voltage close to the ground potential, a lens for
correcting the field curvature may be driven with a voltage
centered at 0 V, and can therefore be driven at high speeds. Axial
chromatic aberration can differ, though slightly, between a value
resulting from a simulation and an actually found value. This
difference can be reduced to zero by adjusting a voltage applied to
aa central electrode 627 of the objective lens 617 and a voltage
applied to one of the upper and lower electrode of the same,
thereby making it possible to more reliably correct chromatic
aberration.
[0647] FIG. 61 is an explanatory diagram illustrating an
electro-optical system of a transfer apparatus of a tenth
embodiment of the electron beam apparatus according to the present
invention. An electron gun comprises a heater 631, a ring-shaped
cathode 632, a Wehnelt 633, and an anode 634. A cross-over formed
by the electron gun is enlarged by condenser lenses 635 and 636 at
two stages, and is irradiated to a sub-visual field on a reticle
637. Electron beams formed by the reticle 637 generates a
one-quarter reduced image (one quarter of the reticle) on an image
plane 641 by axially symmetric magnetic tablet lenses 638 and 640
(640 is an objective lens). A cathode image can be focused on a
back focal plane 642 of the objective lens 640 to produce a hollow
beam and reduce spherical aberration.
[0648] Deflectors 644 and 643 disposed within the lenses 638 and
640 are deflectors for correcting off-axis aberration. These
deflectors can extremely reduce the off-axis aberration, and the
spherical aberration can be reduced by the hollow beam, so that
axial chromatic aberration is major aberration. In this embodiment,
this axial chromatic aberration is corrected by the provision of an
axial chromatic aberration corrector 639. In this way, an image can
be transferred without blur even if the aperture angle is
increased, additionally with an improved throughput. Preferably,
the hollow beam has the aperture angle of 10-11 mrad. This is
because, with a hollow beam having an aperture angle of
.alpha.1-.alpha.2 (mrad), when .alpha.110 mrad and
.alpha.2-.alpha.11 mrad are set, the space charge effect is reduced
so that a transfer can be performed with a large current density.
As illustrated in FIG. 61, the axial chromatic aberration corrector
639 may be disposed below the image plane 641, such that an image
on the image plane 641 is focused on the surface of a sample 645 by
the aberration correction lens 639.
[0649] The correction lens is a Wien filter which has 12 divided
electrodes and magnetic poles, as illustrated in FIG. 61(B), and
generates negative aberration, without generating excessive
aberration, by focusing twice, as indicated by a trajectory 639-1.
Reference numeral 639-2 designates a spacer for insulation.
[0650] FIG. 62 illustrates the structure of an aberration
correction lens system of a chromatic aberration corrector which
can be incorporated in an apparatus which comprises a plurality of
optical systems, i.e., an electron beam apparatus which has a
plurality of optical axes, as illustrated in FIGS. 56-58, FIG. 60,
and FIG. 61. FIGS. 62(A) and 62(B) are a plan view and a
cross-sectional view, respectively. In this example, optical axes
are arranged in two columns and m rows, and one ceramic substrate
is formed with a single-stage quadrupole lens. For manufacturing
this single-stage quadrupole lens, ribs 664 are formed on two
opposing longer edges of the ceramic substrate, four grooves are
radially formed around respective optical axes 659-662, and
respective electrodes 651-654 are formed. Next, the entirety is
non-electrolytically plated, and further plated with Au to form a
coating. Then, the coating is removed from a portion indicated by
shading in FIG. 62(A), and lead lines are connected to side
surfaces of the ribs 664 (in a direction perpendicular to the
figure), as illustrated in FIG. 62(B). In addition, holes 663 are
formed on two opposing shorter edges. In this way, four quadrupole
lenses are formed, and these four lenses are stacked and coupled
such that the holes 663 are in alignment to one another, and are
assembled such that corresponding optical axes are in alignment.
The chromatic aberration corrector thus manufactured can be
employed as a chromatic aberration corrector for an electron beam
apparatus which comprises a barrel or multiple barrels, as
illustrated in FIGS. 56-58 and FIG. 61. Surfaces of the electrodes
651-654 opposing the optical axes 659-662 form part of a hyperbolic
plane.
[0651] FIG. 63 illustrates an electro-optical system of a sample
evaluation apparatus of an eleventh embodiment in the electron beam
apparatus according to the present invention. This apparatus is
configured to eliminate deflection chromatic aberration. Electron
beams from an electron gun 671 are focused at a position 674 by a
condenser lens 673 included in a primary optical system 672 to
focus an image on a sample 678 by an objective lens 677. In this
event, the electron beam is deflected by a deflector 675 and
further deflected by an electromagnetic deflector 676 so as to be
perpendicular to the surface of the sample 678, where the distance
D3 between the focusing position 674 and electrostatic deflector
675 is set to be equal to the distance D4 between the electrostatic
deflector 675 and electromagnetic deflector 676. By setting these
distances to be equal, deflection aberration caused by the two
deflectors can be canceled out by each other to reduce the
deflection aberration to zero as a whole. The electromagnetic
deflector 676 also functions as a beam separator for directing
secondary electron beams to a secondary optical system 679, but can
be simplified in structure because the beam separator can be
implemented by a single electromagnetic deflector. In this regard,
the first embodiment illustrated in FIG. 63 may also comprises an
axial chromatic aberration corrector as is the case with the sixth
to tenth embodiments.
[0652] FIG. 64 illustrates an electro-optical system of a sample
evaluation apparatus of a twelfth embodiment in the electron beam
apparatus according to the present invention. This apparatus
employs three deflectors to eliminate deflection chromatic
aberration. In FIG. 64, electron beams from an electron gun 671 are
focused at a position 674 by a condenser lens 673 included in a
primary optical system 672 to focus an image on a sample 678 by an
objective lens 677. In this event, the electron beams are deflected
by a first to a third deflector 680-682, such that they are
perpendicular to the surface of the sample 678. While the first
deflector 680 may be an electrostatic deflector or an
electromagnetic deflector, the following description will be made
on the assumption that it is an electrostatic deflector. The second
deflector 681 is an electromagnetic deflector, and the third
deflector 682 is an electromagnetic deflector as well.
[0653] As illustrated in FIG. 64, deflection amounts provided by
the first to third deflectors 680-682 are represented by .beta.,
.gamma., .alpha., respectively; the distance between the position
674 and first deflector 680 by D5; the distance between the
position 674 and second deflector 681 by D6; and the distance
between the second deflector 681 and third deflector 683 by D7.
[0654] For imposing electron beams emitted perpendicularly from the
electron gun 671 to impinge perpendicularly on the surface of the
sample 678, the following equation:
.alpha.=.gamma.-.beta. (1)
must be satisfied.
[0655] On the other hand, for eliminating deflection chromatic
aberration caused by the three deflectors, the following
equation:
2.beta.D5=.gamma.D6-.alpha.D7=0 (2)
must be satisfied.
[0656] The ratio of .alpha.:.beta.:.gamma. can be calculated from
the foregoing Equations (1) and (2). For example, when D5=0,
.gamma.D6=.alpha.D7, resulting in:
.alpha./.gamma.=D6/D7 (3)
[0657] Substituting Equation (3) into Equation (1) results in:
.beta./.gamma.=1-D6/D7 (4)
[0658] By thus adjusting the position of the focusing position 674
and deflection angles of the first to third deflectors 680-682,
Equations (1) and (2) can be satisfied. Accordingly, primary
electron beams can be directed perpendicularly into the sample 678,
and the deflection chromatic aberration can be eliminated.
[0659] Advantageously, when primary electron beams are slightly
deflected and secondary electron beams are largely deflected by a
beam separator, aberration does not occur in the primary electron
beams except for the deflection chromatic aberration. Such an
embodiment is illustrated in FIG. 64(B). When the beam separator
682 is implemented by an ExB separator, and similar equations to
the aforementioned Equations (1)-(4) are solved, conditions can be
established for small .alpha. and large .beta., under which the
secondary electron beam can be largely deflected without largely
deflecting the primary electron beams, and the deflection chromatic
aberration can be eliminated.
[0660] It should be noted that the twelfth embodiment illustrated
in FIG. 64 may also comprise an axial chromatic aberration
corrector, as is the case with the sixth to tenth embodiments.
[0661] Incidentally, in a electron beam based sample inspection
apparatus, the resolution is not always important (may be
approximately one fifth to one twentieth of that of a scanning
microscope), but importance is placed on an increase in beam
current for improving the inspection speed. With the employment of
multi-pole lenses at multiple stages as in the present invention,
the beam current can be increased approximately by a factor of ten,
and accordingly, the inspection speed can be improved approximately
by a factor of ten. In the following, the foregoing is described in
greater detail.
[0662] In the projection electron beam apparatus as those in the
sixth to twelfth embodiments, a blur .sigma.c due to the Coulomb
effect can be expressed in the following manner:
.delta.c=IL(.alpha.V.sup.3/2) (5)
where I: Electron Beam Current;
[0663] L: Optical Path Length;
[0664] .alpha.: aperture angle; and
[0665] V: electron beam energy.
[0666] On the other hand, in the projection electron beam
apparatus, axial chromatic aberration is predominate among other
aberration (larger by an order of magnitude as compared with other
aberration), and the other aberration can be kept sufficiently
small by devising the configuration of lenses. Accordingly, the
elimination of the axial chromatic aberration can result in a
reduction in aberration approximately by a factor of ten, and the
aperture angle a can be increased approximately by a factor of ten
in inverse proportion thereto.
[0667] Specifically, the following equation is established:
N=I.eta.t/q
where t represents a time taken to scan one pixel with a beam
current I; q represents an electron charge; N represents the amount
of detected secondary electrons per pixel; and .eta. represents a
second electron emission ratio. For generating a signal
sufficiently larger than shot noise, N must be increased to a
certain value or more, but a certain N value can be ensured as the
beam current I grows, even if the time t is short. Accordingly,
since the scanning time can be shortened, the inspection speed can
be improved.
[0668] Also, in the eighth embodiment which transforms electron
beams emitted from a single electron gun into multiple beams with
the aid of a multi-aperture plate, illustrated in FIG. 58, a
thermoelectron emission scheme is preferably employed with
LaB.sub.6 used for an electron gun. However, this type of electron
gun suffers from chromatic aberration approximately five times
larger as compared with a Schottky type electron gun. This is
because the chromatic aberration in the electron gun depends on an
energy width of electrons from the electron gun, and the energy
width is 0.6 eV in the Schottky type electron gun, and is 3 eV in
the LaB.sub.6 thermoelectron emission type, which is five times
larger than the former.
[0669] Accordingly, in the eighth embodiment, the inspection speed
can be improved by a factor of five with the ability to eliminate
the chromatic aberration, as compared with an apparatus which does
not comprise a chromatic aberration correction function. Further,
it should be understood that the inspection speed can be largely
improved by applying the technical idea of the present invention to
an electron beam apparatus which comprises a plurality of
single-beam or multi-beam projection barrels.
[0670] As described above, when the technical idea of the present
invention is applied to a sample evaluation apparatus and a
lithography apparatus, the chromatic aberration can be corrected so
that the beam diameter can be increased, thus making it possible to
perform processing at a high throughput. Also, when the technical
idea of the present invention is applied to a sample evaluation
apparatus, the space charge effect can be reduced, so that the
throughput is further improved.
[0671] FIG. 65(A) generally illustrates the configuration of a
thirteenth embodiment of the electron beam apparatus according to
the present invention. In FIG. 65(A), the electron beam apparatus
comprises an electron gun 691, a primary electro-optical system
692, a beam separation system 693, an objective optical system 694,
a secondary electro-optical system 695, and a secondary electron
detection system 696.
[0672] Electron beams emitted from the electron gun 691 are
enlarged by a first condenser lens 697 and a second condenser lens
698 in the primary electro-optical system 692, and are irradiated
to an aperture plate 699 having a rectangular formation aperture.
With this aperture plate, primary beams are formed to have a
rectangular cross-section. The primary beams, rectangular in
cross-section, is formed by a formation lens 700, and adjusted in
magnification by a variable magnification lens 701, before they are
directed into the beam separation system 693. The beam separation
system 693 is, for example, an ExB separator which comprises an
electrostatic deflector 702 and an electromagnetic deflector 703.
The primary beams incident on the beam separation system 693
changes their traveling direction in a direction perpendicular to a
sample W.
[0673] The primary beams, the traveling direction of which has been
changed to the direction perpendicular to the sample W by the beam
separation system 693 enter the objective optical system 694. The
objective optical system 694 comprises an NA aperture plate 704
having a ring-shaped aperture and a rectangular aperture through
which the primary beams pass; a dynamic focusing electrode 705; a
high voltage application electrode 706; and an objective lens
electrode 707. The NA aperture plate 704 has a plurality (four in
FIG. 65(B)) of elongated holes 715, 716, 717, 178 which are
arranged on the same circumference to create a ring-shaped
aperture, and a rectangular hole 719 for passing the primary beams
therethrough. The primary beams in rectangular cross-section, which
have passed through the beam separation system 693 are deflected to
pass through the rectangular hole 719. Subsequently, the primary
beams are focused on the sample W by the dynamic focusing electrode
705, high voltage application electrode 706, and objective lens
electrode 707, and irradiated to the surface of the sample W.
[0674] Secondary electrons emitted from the sample W, which has
been irradiated with the primary beams in rectangular cross-section
are accelerated and converged by a high voltage generated by the
objective lens electrode 707, and intersect with the optical axis
of the secondary electrons at the position of the NA aperture plate
704, and form an enlarge image at a position P. The position at
which the NA aperture plate 704 is disposed is, as confirmed by a
simulation, a position at which a total of coma aberration and
magnification chromatic aberration is minimized. While the
conventional NA aperture plate is formed with a single circular
hole, it has suffered from a problem that an image blurs more due
to the space charge effect. Thus, in the present invention, the
secondary electrons are transformed into hollow beams by the
ring-shaped holes 715-718 of the NA aperture plate 704, and these
beams alone are directed toward the detection system 696. Since the
holes 715-718 of the NA aperture plate 704 have a small width D,
spherical aberration is sufficiently small in the secondary
electro-optical system 695. However, since beams with a large
aperture angle are used in such an electron beam apparatus, axial
chromatic aberration can constitute a grave problem. As such, the
hollow secondary electrons separated from the primary electrons by
the beam separation system 695 and directed to the detection system
96 are corrected for axial chromatic aberration by an axial
chromatic aberration correction lens 708 composed of quadrupole
lenses at a plurality (four in this embodiment) of stages in the
secondary electro-optical system 695.
[0675] The secondary electrons, which have been corrected for the
axial chromatic aberration, are enlarged by an auxiliary lens 710
disposed at an image point 709 of the axial aberration correction
lens 708, and a magnification lens 711 disposed on the downstream
side of the auxiliary lens 710, and is further enlarged by an
auxiliary lens 712 and a magnification lens 713 disposed on the
downstream side. The auxiliary lens 712 forms reduced images of the
ring-shaped holes 715-718 of the NA aperture plate 704 on the main
surface of the magnification lens 713. The secondary electron image
thus formed is enlarged by the magnification lens 713, and the
enlarged image is focused on a MCP (micro-channel plate) 714 of the
detection system 696. The detection system 696 is used to evaluate
the sample W for defects using the image projected by the
magnification lens 713.
[0676] FIG. 66(A) is a diagram generally illustrating the
configuration of a fourteenth embodiment of the electron beam
apparatus according to the present invention, which comprises, like
the first embodiment, an electron gun 721, a primary
electro-optical system 722, a beam scanning/separation system 723,
an objective lens 724, a secondary electro-optical system 725, and
a secondary electron detection system 726. In FIG. 66, the electron
gun 721 comprises a cathode 731 and a Wehnelt electrode 732, where
the cathode 731 comprises a cylindrical material made of an
LaB.sub.6 single crystal formed with a ring-shaped edge in one end
surface thereof, as illustrated. Accordingly, the cathode 731 emits
electron beams in hollow cross-section. The Wehnelt electrode 732
surrounds the cathode 731, and is applied with such a voltage that
forms a cross-over image Cl between the electrode and a first
condenser lens 733 of the primary electro-optical system 2.
[0677] The primary electro-optical system 722 comprises a first and
a second condenser lens 733, 734, a multi-aperture plate 735, a
reducing lens 736, and an axial chromatic aberration correction
lens 737. The cross-over image C1 formed by the electron gun 721 is
enlarge by the first condenser lens 733 and second condenser lens
734, which are magnification lenses, at two stages, and is
uniformly irradiated to the multi-aperture plate 735. It should be
noted that the second condenser lens 734 is adjusted in excitation
such that the multi-aperture plate 735 is widely and uniformly
irradiated, and such that a cross-over image C2 of the condenser
lens 734 comes slightly closer to the condenser lens 734 than the
multi-aperture plate 735. The multiple primary beams produced by
the multi-aperture plate 735 are scaled down by the reducing lens
736, and enters the beam scanning/separation system 723, with
negative axial chromatic aberration caused by the axial chromatic
aberration correction lens 737.
[0678] The beam scanning/separation system 723 comprises a scanning
deflector 738, an electromagnetic deflector 739, and an
electrostatic deflector 740. The primary beams having the negative
axial chromatic aberration are changed in traveling direction by
the scanning deflector 738 such that they go to the electromagnetic
deflector 739. The traveling direction is again changed by the
electromagnetic deflector 739 such that the primary beams impinge
vertically on the sample W. In this event, deflection chromatic
aberration can occur in the primary beam in the beam
scanning/separation system 723. Accordingly, the scanning deflector
738 is positioned at the midpoint between a cross-over image C3 of
the axial chromatic aberration lens 737 and the beam
scanning/separation system 723, such that the deflection chromatic
aberration is corrected by deflecting the primary beams by the
scanning deflector 738 and electromagnetic deflector 739 by the
same angle in directions opposite to each other. The beams having
the negative axial chromatic aberration are canceled out by
positive axial chromatic aberration possessed by the objective
lens, to correct the axial chromatic aberration. The electrostatic
deflector 740 is used to scan the multiple beams on the sample.
[0679] Now, the axial chromatic aberration correction lens 737 will
be described with reference to FIG. 66(B). This correction lens 737
is also referred to as a "Wien filter" and converges beams emitted
from an end surface twice, but causes negative axial chromatic
aberration in the cross-over image C3 due to non-dispersion. FIG.
66(B) illustrates one quarter of the cross-section of the
correction lens 737. As can be understood from FIG. 66(B), the
correction lens 737 has 12 poles, where the Wien condition is
satisfied by a dipole field, and the axial chromatic aberration is
made negative by quadrupole electric field/magnetic field, and
hexipolar electric field/magnetic field is applied to generate
negative spherical chromatic aberration, thus making it possible to
partially correct spherical aberration which mainly occurs in the
objective lens 742. When the spherical aberration is larger, a
majority of the spherical aberration and part of the axial
chromatic aberration may be corrected.
[0680] A 12-pole electrode 746 is made of permalloy B, and
generates a dipole, a quadrupole, and a hexipolar magnetic field by
applying a current to a coil 747. In the figure, reference numeral
749 designates a core made of permalloy, and 748 designates a
spacer for insulating each electrode.
[0681] The primary beams which have passed through the beam
scanning/separation system focuses an image of the cathode 31 on
the NA aperture plate 741 in the objective optical system 724. This
is implemented by adjusting the reducing lens 736. For this
purpose, the NA aperture plate 741 has a hole large enough to pass
hollow beams therethrough. Multiple beams which have been formed by
the multi-aperture plate 735 and scaled down by the reducing lens
736 are again scaled down by the objective lens 742, before they
are irradiated to the sample W. In this event, the surface of the
sample W can be scanned with the primary beams by applying a
scanning signal to the scanning deflector 738 and auxiliary
deflector 740. The fulcrum of deflection at this time is found at
the position of the aperture on the NA aperture plate 41.
[0682] Multiple secondary beams are emitted from the sample W
irradiated with the primary beams. The emitted secondary beams are
accelerated by a high voltage of the objective lens 742, passes
through the apertures of the NA aperture plate 741, are separated
by the electromagnetic deflector 739 from the primary beams, and
travels toward the secondary electro-optical system 725. The
secondary electro-optical system 725 comprises a plurality (two in
this embodiment) of magnification lenses 743, 744, so that the
secondary beams are enlarged by these magnification lenses 743, 744
to focus an image on a multi-detector 7454 in the detection system
726. In this event, zoom lenses may be employed for the
magnification lenses 743, 744, whereby the spacings between a
plurality of detectors which make up the multi-detector 745 can be
precisely matched with the spacing between images of multiple beams
which make up the secondary beams, as well as the secondary beams
can be detected without changing the spacing between the detectors
when the unit area of the sample W irradiated with the secondary
beams, i.e., the dimensions of the pixels are increased by a factor
of two, four and the like and reduced by a factor of two, four and
the like.
[0683] FIG. 67 is a diagram generally illustrating the
configuration of a fifteenth embodiment of the electron beam
apparatus according to the present invention, where the electron
beam apparatus comprises an electron gun 751, a primary
electro-optical system 752, a beam scanning/separation system 753,
an objective optical system 754, and a secondary electro-optical
system 756.
[0684] The electron gun 751 comprises a cathode 761, a Wehnelt
electrode 762, and an anode 763. The cathode 761 comprises a
cylindrical material made of a LaB.sub.6 single crystal which has
one end surface polished to form a ring-shaped knife edge. A ring
formed by the knife edge has a diameter of 0.6 mm, by way of
example. Thus, hollow electron beams are emitted from the cathode
761. The periphery of the cathode 761 is surrounded by the Wehnelt
electrode 762, and the Wehnelt electrode 762 is biased negatively
with respect to the cathode 761 such that a virtual cross-over
image C4 is formed behind the cathode 761. The surface of the
Wehnelt electrode 762 opposite to the end surface of the cathode
761 formed with the knife edge, is a flat electrode formed with a
hole for passing primary beams emitted from the cathode 761
therethrough. The hole has a diameter of 3 mm, by way of example,
and the spacing of the leading edge of the cathode 761, i.e., the
leading edge of the knife edge, and the flat electrode of the
Wehnelt electrode 762 is, for example, 300 .mu.m.
[0685] Primary beams emitted from the electron gun 751 are
processed by the primary electro-optical system 2 which comprises a
first reducing lens 764, a second reducing lens 765, and an axial
chromatic aberration correction lens 766, and enters the beam
scanning/separation system 753. Describing in greater detail, the
virtual cross-over image C4 is scaled down by the first reducing
lens 764 and second reducing lens 765, and the second reducing lens
765 forms a cross-over image C5. The axial chromatic aberration
correction lens 766 comprises a first quadrupole lens 767, a second
quadrupole lens 768, a third quadrupole lens 769, a fourth
quadrupole lens 770, a first quadrupole magnetic lens 771, and a
second quadrupole magnetic lens 772, and forms a cross-over image
C6 which is corrected for axial chromatic aberration of the
cross-over image C5. The primary beams which have formed the
cross-over image C6 in this way enter the beam scanning/separation
system 753.
[0686] The beam scanning/separation system 753 comprises an
electromagnetic deflector 774 and an electrostatic deflector 775.
The primary beams are changed in traveling direction by the
scanning deflector 773, and the traveling direction is again
changed by the electromagnetic deflector 774 such that the primary
beams impinge perpendicularly to the sample W. In this event,
deflection chromatic aberration can occur in the primary beams due
to the beam scanning/separation system 753. Accordingly, the
scanning deflector 773 is positioned at the midpoint between the
cross-over image C6 of the axial chromatic aberration lens 766 and
the beam scanning/separation system 753, such that the deflection
chromatic aberration is corrected by deflecting the primary beams
by the scanning deflector 773 and electromagnetic deflector 775 by
the same angle in directions opposite to each other.
[0687] The primary beams which have passed the beam
scanning/separation system 753 focus an image of the cathode 761 on
an NA aperture plate 776 of the objective optical system 4. This is
implemented by adjusting the reducing lens 765. By focusing the
image of the cathode on the NA aperture plate 776, the primary
beams are hollow at the position of the NA aperture plate 776. The
cross-over having negative axial chromatic aberration appear to be
hollow beams at the position of the NA aperture plate, and are
scaled down by the objective lens 777, before they are irradiated
to the sample W. In this event, the primary electro-optical system
is preferably designed such that the aperture angle to the image of
the cathode 761, viewed from the sample W, is 100 mrad, by way of
example. This aperture angle of 100 mrad can cause a problem of
spherical aberration when the primary beams are solid beams, but
since the primary beams are hollow beams in the present invention,
the spherical aberration can be neglected if the ring has a small
width.
[0688] The virtual cross-over C4 is scaled down, for example, to
approximately 1/1000 by the reducing lenses 764, 765 at two stages
and the objective lens 777. Here, the surface of the sample W can
be scanned with the primary beams by applying a scanning signal to
the scanning deflector 773 and electrostatic deflector 775. The
fulcrum of deflection at this time is located at the position of
the aperture on the NA aperture plate 776.
[0689] Secondary electrons emitted from the sample W irradiated
with the primary beams are accelerated by a high voltage of the
objective lens 777, pass through the aperture of the NA aperture
plate 776, are separated from the primary beams by the beam
scanning/separation system 753, and enter the detection system 756.
The detection system 756 may comprise, for example, an SE detector
778.
[0690] As will be understood from the foregoing description, in the
thirteenth to fifteenth embodiments, the beams on the sample W are
corrected for axial chromatic aberration, and the blur due to the
space charge effect is insignificant, so that beams of small
diameters can be formed with a large beam current.
[0691] It should be noted that devices can be manufactured by
utilizing the electron beam apparatus described in connection with
FIGS. 56 to 70 in the manufacturing process illustrated in FIGS. 53
and 54. Specifically, when the electron beam apparatus according to
the present invention is used for the chip testing process at step
403 for performing a defect test, even semiconductor devices having
fine patterns can be tested at a high throughput, thus making it
possible to enable a total inspection as well as to improve the
yield rate of products, and prevents the shipment of defective
products.
[0692] As will be understood from the foregoing description, the
present invention advantageously reduces the influence of the space
charge effect, resulting from primary beams which are produced into
a hollow by the provision of an aperture plate which has apertures
in a ring shape.
[0693] FIG. 68 is a diagram generally illustrating the
configuration of a sixteenth embodiment of the electron beam
apparatus according to the present invention. This electron beam
apparatus comprises an electron beam emitter 781, a primary
electro-optical system 782, a beam separation system 783, an
objective optical system 784, a secondary electro-optical system
785, and a detection system 786. The electron emitter 781 comprises
an electron gun 790 which has a cathode made of single crystal
LaB.sub.6 and operates under a space charge limiting condition.
[0694] Primary beams emitted from the electron gun 790 enter the
primary electro-optical system 782. The primary electro-optical
system 782 comprises a condenser lens 791, a multi-aperture plate
792, a rotation correction lens 793, an NA aperture plate 794, a
reducing lens 795, and an axial chromatic aberration correction
lens 796. The primary beams from the electron gun 790 are converged
by the condenser lens 791, and are uniformly irradiated to the
multi-aperture plate 792 which has a plurality of apertures. The
electron beams transformed into multiple beams by the
multi-aperture plate 792 focus a cross-over on the NA aperture
plate 794 by the condenser lens 791 and rotation correction lens
793. With the cross-over fixed on the NA aperture plate 794, the
condenser lens 791 and rotation correction lens 793 can adjust an
irradiated region of the multi-aperture plate 792, or adjust a
current density at which the multi-aperture plate 792 is
irradiated.
[0695] The multiple beams which have passed through the NA aperture
plate 794 are scaled down by the reducing lens 795 to form an image
of the multi-aperture plate 792 at a point 797. This image is
transformed into an image 798 of the multi-aperture plate 792
having negative axial chromatic aberration by the axial chromatic
aberration correction lens 796. The multiple beams which have
formed the image 798 is changed in traveling direction to a
direction perpendicular to the sample W by the beam separation
system 783 which comprises an electromagnetic deflector 800 and an
electrostatic deflector 801, and further converged by the objective
lens 802 to form a final image on the sample W. It should be noted
that the aforementioned negative axial chromatic aberration is
canceled out by positive axial chromatic aberration of an objective
lens (later described), so that chromatic aberration is
eliminated.
[0696] The axial chromatic aberration correction lens 796 comprises
quadrupole lenses QL1, QL2, QL3, QL4 at a plurality of, for
example, four stages, and magnetic quadrupoles 803, 804, and is
designed to cancel out axial chromatic aberration caused by the
reducing lens 795 and objective lens 802 with the aid of the
quadrupole lenses QL1-QL4 and magnetic quadrupoles 803, 804. Also,
the scanning of the sample W by the multiple beams is served by
electrostatic deflectors 799, 801 at two stages, and particularly,
aberration can be reduced during the scanning by optimizing the
ratio of a scanning signal applied to the electrostatic deflector
799 to a scanning signal applied to the electrostatic deflector 801
to optimize the fulcrum of deflection. Here, the deflection
aberration of the beam separator can be substantially completely
corrected by setting the distance between the reduced image 798 of
the multi-aperture plate 792 and the electrostatic deflector 799 to
one-half of the distance between the image 798 and electromagnetic
deflector 800.
[0697] Second electrons emitted from a scanned point of the sample
W are accelerated by a high voltage of the objective lens 802 and
separated from the primary beams by the electromagnetic deflector
800, and enter the secondary electro-optical system 785. The
secondary electro-optical system 785 comprises a magnification lens
805 and rotation correction lenses 806, 807. The secondary electron
beams separated by the electromagnetic deflector 800 are enlarged
by the magnification lens 805, and are again enlarged by the
rotation correction lenses 806, 807 to form an enlarged image in
the detection system 6. The detection system 786 comprises multiple
detectors 808 which are arranged on the same plane, and can thus
detect each of multiple beams independently by each detector. The
rotation correction lenses 806, 807 are of current control type,
and currents are controlled such that axial magnetic fields are
generated in opposite directions to each other.
[0698] FIG. 69 is a beam arrangement diagram showing how beams are
arranged in a region of the sample W irradiated with the multiple
beams formed from the primary beams by the multi-aperture plate 792
in FIG. 68, where the position of each beam is indicated by a black
circle. In FIG. 69, a region surrounded by a circle 811 indicates a
region in which the multi-aperture plate 792 is uniformly
irradiated with beams emitted from the electron gun 790, or a
region in which aberration of the optical system is equal to or
less than a specified value, and its diameter d1 is, for example, 4
.mu.m. Specifically, in this region, 90% or more of beam intensity
can be provided with respect to the beam intensity on the optical
axis.
[0699] The region is irradiated with the intensity of 90% or more
because the formation of an image is hindered if the multiple beams
are not uniform in intensity. It is further necessary to reduce
aberration of the optical system to a predefined value or less in
order to narrow down all the multiple beams. While it is known that
the axial chromatic aberration can be corrected by a correction
lens, the optical system is not axially symmetric and is therefore
expected to have much off-axis aberration as well. Accordingly, a
circle 811 may represents a region in which the off-axis aberration
of the axial chromatic aberration correction lens 796 is smaller
than a predetermined threshold. Actually, these conditions must be
satisfied.
[0700] As illustrated, when coordinates having two orthogonal axes
X, Y are defined on the region, the multiple beams are arranged in
a matrix of m rows and n columns on the XY-plane (where m and n are
positive integers, m represents the number of beams in the X-axis
direction, and n represents the number of beams in the Y-axis
direction). An interval d2 between the respective columns is, for
example, 403 nm. In this way, m*n beams are created in the region
811. Then, when the sample W is scanned in the X-axis direction,
the distance d3 between the respective beams, when projected onto
the Y-axis, can all be made equal by inclining the scanning
direction by sin.sup.-1(1/m) with respect to the X-axis. For
example, when m=8, sin.sup.-1(1/8)=7.18 degrees, and d3 is, for
example, 50 nm. In this way, multi-channel SEM images can be formed
without waste by setting the raster pitch when scanning in the
X-axis direction, i.e., the distance between adjacent trajectories
when one beam scans the sample W to be equal to the dimension of
one pixel or an integer multiple thereof.
[0701] The beam interval d2 is equal to pixel
dimension*m/cos(sin.sup.-1(1/m)), where larger m causes the
denominator to approach to one, and the beam interval to be equal
to the pixel dimension multiplied by m. Here, as m is larger, the
beam interval is wider, which is disadvantageous for arranging many
beams within a circle in which the aberration falls within a
certain level. Conversely, with a larger intervals of secondary
electron beams, the multi-detector 808 more readily detects the
beams. In this sense, when the number of beams is preferred, m<n
should be satisfied. Conversely, when the ease of detecting the
beams is preferred, m>n should be satisfied.
[0702] It should be noted that even when mn, the raster interval
can be made equal when additional beams 812-821 are further
arranged outside the matrix of m rows and n columns, as illustrated
in FIG. 69.
[0703] As described above in detail, in the sixteenth embodiment of
the present invention, since the axial chromatic aberration of the
primary beams is corrected by the use of the axial chromatic
aberration correction lens 796, the S/N ratio can be accomplished
as required to perform evaluations at a high resolution even if the
throughput is increased. In addition, since the electromagnetic
deflector 800 is disposed between the reduced image 798 of the
multi-aperture plate 792 and the objective lens 802, the primary
beams and secondary electrons commonly pass over a reduced
distance, so that the secondary beams exert reduced space charge
effects on the primary beams. Consequently, the sample W can be
scanned with a large number of narrowed beams.
[0704] Next, a seventeenth embodiment of the electron beam
apparatus according to the present invention will be described with
reference to FIG. 70. Likewise, in this embodiment, the electron
beam apparatus comprises an electron beam emitter 831, a primary
electro-optical system 832, a beam separation system 833, an
objective optical system 834, a secondary electro-optical system
835, and a detection system 836. Electron beams emitted from an
electron gun 840 of the electron beam emitter 831 are converged by
a condenser lens 841 of the primary electro-optical system 832, and
irradiated to a formation aperture plate 842 with a uniform
intensity (for example, within 20% of intensity non-uniformity). In
this way, the electron beams which have passed through the
formation aperture plate 842 are formed into beams having a
rectangular cross-section.
[0705] The primary beams thus formed to have a rectangular
cross-section passes through rotation correction lenses 843, 844, a
formation lens 845, and alignment deflectors 846, 847, which also
make up the primary electro-optical system 832, and enter the beam
separation system 833. The beam separation system 833 comprises an
electromagnetic deflector 848. The primary beams which have passed
through the primary electro-optical system 832 are changed in
traveling direction to a direction perpendicular to the sample W by
the electromagnetic deflector 848. The primary beams, the traveling
direction of which has been changed by the electromagnetic
deflector 848, pass through an aperture for the primary beams
provided through an NA aperture plate 849 of the objective lens
system 834, and converged and focused on the sample W by an
objective lens 850.
[0706] The alignment deflectors 846, 847 are provided for shifting
the primary beams such that the primary beams and secondary beams
emitted from the sample irradiated with the primary beams take
different trajectories between the electromagnetic deflector 848
and sample W. In FIG. 70, a dotted line 851 represents the
trajectory of the primary beams shifted by these alignment
deflectors 846, 847. In FIG. 70, the trajectory of the secondary
electrons are enlarged in the lateral direction so that the primary
electrons appear to pass within the secondary beams, but actually,
the primary beams pass outside the secondary beams. Also, as
illustrated in FIG. 70, a plurality of apertures are provided
through the formation aperture plate 842 in order to compensate for
a change in magnification due to a change in the acceleration
voltage intended to change an irradiation voltage. Further, the two
rotation correction lenses 843, 844 are used to correct for a
rotation amount because the primary beams formed to have a
rectangular cross-section by the formation aperture plate 842
change a rotating angle due to a change in the acceleration voltage
by a variety of lenses of the primary electro-optical system
832.
[0707] The secondary beams emitted from the sample W irradiated
with the primary beams pass through the objective lens 850 and NA
aperture plate 849, are separated from the primary beams by the
electromagnetic deflector 848, and deflected by an electrostatic
deflector 852 in the beam separation system 833 to form an enlarged
image at a point 853. The electrostatic deflector 852 is provided
to deflect by the same angle as and the direction opposite to the
electromagnetic deflector 848 in order to correct the deflection
chromatic aberration caused by the electromagnetic deflector 848.
For correcting the deflection chromatic aberration, the distance
between the point 853 and electromagnetic deflector 848 is set to
be twice the distance between the point 853 and electrostatic
deflector 852.
[0708] The secondary beams which have formed the enlarged image at
the point 853 by the objective lens 849 are corrected for axial
chromatic aberration by an axial chromatic aberration correction
lens 854 which comprises a non-dispersion Wien filter, and are
focused on a main surface of an auxiliary lens 855. Then, the
auxiliary lens 855 focuses the image of the NA aperture plate 849
on a main surface of a magnification lens 856 as an image 857,
thereby reducing the divergence of the beams by the magnification
lens 856 to reduce distortion aberration caused by the
magnification lens 856. The image of the NA aperture plate 849 is
enlarged by magnification lenses 856, 858 to form an enlarged image
on an MCP (micro-channel plate) 859 of the detection system 836. In
this way, the image of the sample W is detected. It should be noted
that an auxiliary lens 860 is disposed at the image point of the
enlarge image by the magnification lens 856. The auxiliary lens 860
has a function of forming the image 857 of the NA aperture plate
849 on a main surface of the magnification lens 858.
[0709] Now, the axial chromatic aberration correction lens 854 will
be described. This correction lens 854 is also referred to as a
"Wien filter" and converges beams emitted from an end surface
twice, but causes negative axial chromatic aberration in the
cross-over image C3 due to non-dispersion. FIG. 71 illustrates one
quarter of the cross-section of the correction lens 854. As can be
understood from FIG. 71, the correction lens 854 has 12 poles,
where the Wien condition is satisfied by a dipole field, and the
axial chromatic aberration is made negative by quadrupole electric
field/magnetic field, and hexipolar electric field/magnetic field
is applied to generate negative spherical chromatic aberration,
thus making it possible to partially correct spherical aberration
which mainly occurs in the objective lens 850. A 12-pole electrode
854-1 is made of permalloy B, and generates a dipole, a quadrupole,
and a hexipolar magnetic field by applying a current to a coil
854-2. In the figure, reference numeral 854-3 designates a core
made of permalloy, and 854-4 designates a spacer for insulating
each electrode.
[0710] As described above, since the secondary beams have been
corrected for the axial chromatic aberration in the secondary
electro-optical system 5, the aberration is reduced even if the NA
aperture plate 849 is increased, so that secondary beams having a
large aperture angle can pass through the NA aperture plate 849.
Consequently, the transmittance of the secondary beams is high and
more secondary beams per pixel enter the MCP 859, advantageously
making it possible to process images at high speeds.
[0711] In FIG. 70, reference numeral 757 designates an image of the
NA aperture plate 849 focused on the main surface of the
magnification lens 856. The position of an optical conjugate plane
857 of the NA aperture plate 849 can be set at an arbitrary
position along the optical axis of the secondary beams by adjusting
the focal distance of the auxiliary lens 855. Even if such a
setting is made, the image of the secondary beams corrected for the
axial chromatic aberration is formed on the main surface of the
auxiliary lens 855. The position of the NA aperture plate 849 is
designed by taking advantage of the fact that secondary beams
emitted from an end of the visual field intersect with the optical
axis at a position slightly deviated from a position at which
secondary beams emitted from the vicinity of the optical axis
intersect with the optical axis. By designing the NA aperture plate
849 such that it is installed at the position at which the
secondary beams emitted from an end intersect with the optical
axis, a signal based on the secondary beams emitted from an end of
the visual field can be enhanced, thereby partially solving a
problem that the electron gun cannot increase the beam current
density at ends of the visual field.
[0712] On the other hand, in regard to the shape of the visual
field, the axial chromatic aberration correction lens 854 cannot
expand the visual field. For this reason, the surface of the sample
W is illuminated by the primary beams in a circular shape, and the
MCP 859 preferably has a square detection plane of 2048*2048
pixels, by way of example.
[0713] In the two embodiments, when the objective lenses 802, 850
are electromagnetic lenses, secondary electron beams emitted in the
normal direction of the sample W do not intersect with the optical
axis. However, it has been found from a simulation that secondary
electron beams emitted in a direction inclined by 8.5 degrees from
the normal direction intersect with the optical axis. Since the
secondary electron beams are emitted in accordance with the cosine
law, the secondary electron beams emitted in the direction inclined
by 8.5 degrees has an intensity equal to cos 8.5=0.9 which is not
so different from that of secondary electron beams emitted in the
axial axis direction. Accordingly, in the present invention,
required operations can be performed taking advantage of the
secondary electron beams emitted in the direction inclined by 8.5
degrees from the normal direction of the sample W.
[0714] While the electron beam apparatus according to the present
invention, and a method of manufacturing a semiconductor device
using the apparatus have been described above, the present
invention is not limited to their embodiments, but can be modified
and altered in various manners, as is obvious to those skilled in
the art. For example, as detectors for use in the detection system
6, line sensors may be used instead of plane sensors such as the
MCP, multi-detector and the like. In the present invention, since
the primary electro-optical system or secondary electro-optical
system is provided with an axial chromatic aberration correction
lens, a sample can be evaluated at a largely improved throughput
and a high resolution. While the axial chromatic aberration
correction lens has a relatively narrow visual field,
two-dimensional images of a sample can be efficiently captured by
employing a visual field having a two-dimensional extent, for
example, a square visual field. Also, since the primary beams and
secondary beams pass over a short common distance, another effect
produced therefrom is that the influence of the space charge effect
is mitigated.
[0715] While preferred embodiments of the present invention have
been described above in detail, it is apparent that these
embodiments can be altered and modified without departing from the
technical idea of the present invention.
* * * * *